CBSE Class 8

These NCERT Solutions for Class 8 Maths Chapter 1 Rational Numbers Ex 1.2 Questions and Answers are prepared by our highly skilled subject experts.

NCERT Solutions for Class 8 Maths Chapter 1 Rational Numbers Exercise 1.2

Question 1.
Represent these numbers on the number line:
(i) \(\frac{7}{4}\)
(ii) \(\frac{-5}{6}\)
Solution:
(i) \(\frac{7}{4}\)
We make 7 markings at distance of \(\frac{1}{4}\) each on the right of 0 and starting from 0, the seventh marking represents \(\frac{7}{4}\).

(ii) \(\frac{-5}{6}\)
We make 5 markings at distance of \(\frac{1}{6}\) each on the left of ‘0’ and starting from ‘0’, the fifth marking represents \(\frac{-5}{6}\)

Question 2.
Represent \(\frac{-2}{11}, \frac{-5}{11}, \frac{-9}{11}\) on a number line.
Solution:
We make 9 markings at distance of \(\frac{1}{11}\) each on the left of ‘0’ and starting from ‘0’, the second marking represents \(\frac{-2}{11}\); the fifth marking represents \(\frac{-5}{11}\) and the ninth marking represents \(\frac{-9}{11}\).
The point A represents \(\frac{-2}{11}\), the point B represents \(\frac{-5}{11}\) and the point C represents \(\frac{-9}{11}\).

Question 3.
Write five rational numbers which are smaller than 2.
Solution:
There can be infinite rational numbers smaller than 2.
Five rational numbers are 0, -1, \(\frac{1}{2}\), \(\frac{-1}{2}\), 1

Question 4.
Find ten rational numbers between \(\frac{-2}{5}\) and \(\frac{1}{2}\)
Solution:
Convert \(\frac{-2}{5}\) and \(\frac{1}{2}\) with the same denominators.
\(\frac{1}{2}=\frac{1 \times 5}{2 \times 5}=\frac{5}{10}\)
\(\frac{-2}{5}=\frac{2 \times 2}{5 \times 2}=\frac{-4}{10}\)
To get ten rational numbers, multiply both numerator and denominator by 2
\(\frac{5}{10}=\frac{5 \times 2}{10 \times 2}=\frac{10}{20}\)
\(\frac{-4}{10}=\frac{-4 \times 2}{10 \times 2}=\frac{-8}{20}\)
The rational numbers between \(\frac{10}{20}\) and \(\frac{-8}{20}\) are
\(\frac{9}{20}, \frac{8}{20}, \frac{7}{20}, \frac{6}{20}, \frac{5}{20}, \frac{4}{20}, \frac{3}{20}, \ldots \frac{-6}{20}, \frac{-7}{20}\)
We can take any 10 of them.

Question 5.
Find five rational numbers between
(i) \(\frac{2}{3}\) and \(\frac{4}{5}\)
(ii) \(\frac{-3}{2}\) and \(\frac{5}{3}\)
(iii) \(\frac{1}{4}\) and \(\frac{1}{2}\)
Solution:
Convert \(\frac{2}{3}\) and \(\frac{4}{5}\) with the same denominators
\(\frac{2}{3}=\frac{2 \times 5}{3 \times 5}=\frac{10}{15}\)
\(\frac{4}{5}=\frac{4 \times 3}{5 \times 3}=\frac{12}{15}\)
The difference between the numerators should be more than 5
\(\frac{10}{15}=\frac{10 \times 4}{15 \times 4}=\frac{40}{60}\)
\(\frac{12}{15}=\frac{12 \times 4}{15 \times 4}=\frac{48}{60}\)
Five rational numbers between \(\frac{40}{60}\) and \(\frac{48}{60}\) are
\(\frac{41}{60}, \frac{42}{60}, \frac{43}{60}, \frac{44}{60}, \frac{45}{60}\)

(ii) Convert \(\frac{-3}{2}\) and \(\frac{5}{3}\) with the same denominators.
\(\frac{-3}{2}=\frac{-3 \times 3}{2 \times 3}=\frac{-9}{6}\)
\(\frac{5}{3}=\frac{5 \times 2}{3 \times 2}=\frac{10}{6}\)
Five rational numbers between \(\frac{-9}{6}\) and \(\frac{10}{6}\) are \(\frac{9}{6}, \frac{8}{6}, \frac{7}{6}, 0, \frac{-7}{6}\)

(iii) Convert \(\frac{1}{4}\) and \(\frac{1}{2}\) with the same denominators
\(\frac{1}{4}=\frac{1}{4}\)
\(\frac{1}{2}=\frac{1 \times 2}{2 \times 2}=\frac{2}{4}\)
The difference between the numerators should be more than 5
\(\frac{1}{4}=\frac{1 \times 8}{4 \times 8}=\frac{8}{32}\)
\(\frac{2}{4}=\frac{2 \times 8}{4 \times 8}=\frac{16}{32}\)
Five rational numbers between \(\frac{8}{32}\) and \(\frac{16}{32}\) are \(\frac{9}{32}, \frac{10}{32}, \frac{11}{32}, \frac{12}{32}, \frac{13}{32}\)

Question 6.
Write five rational numbers greater than -2.
Solution:
There are infinite rational numbers greater than -2.
The numbers are \(\frac{-3}{2},-1, \frac{-1}{2}, 0, \frac{1}{2}\)

Question 7.
Find ten rational numbers between \(\frac{3}{5}\) and \(\frac{3}{4}\).
Solution:
Convert \(\frac{3}{5}\) and \(\frac{3}{4}\) with the same denominators
\(\frac{3}{5}=\frac{3 \times 4}{5 \times 4}=\frac{12}{20}\)
\(\frac{3}{4}=\frac{3 \times 5}{4 \times 5}=\frac{15}{20}\)
The difference between the numerator should be more than 10.
\(\frac{12}{20}=\frac{12 \times 8}{20 \times 8}=\frac{96}{160}\)
\(\frac{15}{20}=\frac{15 \times 8}{20 \times 8}=\frac{120}{160}\)
Thus, ten rational numbers between

These NCERT Solutions for Class 8 Maths Chapter 1 Rational Numbers Ex 1.1 Questions and Answers are prepared by our highly skilled subject experts.

NCERT Solutions for Class 8 Maths Chapter 1 Rational Numbers Exercise 1.1

Question 1.
Using appropriate properties, find:
(i) \(\frac{-2}{3} \times \frac{3}{5}+\frac{5}{2}-\frac{3}{5} \times \frac{1}{6}\)
(ii) \(\frac{2}{5} \times\left(\frac{-3}{7}\right)-\frac{1}{6} \times \frac{3}{2}+\frac{1}{14} \times \frac{2}{5}\)
Solution:

Question 2.
Write the additive inverse of each of the following:
(i) \(\frac{2}{8}\)
(ii) \(\frac{-5}{9}\)
(iii) \(\frac{-6}{-5}\)
(iv) \(\frac{2}{-9}\)
(v) \(\frac{19}{-6}\)
Solution:
(i) Additive inverse of \(\frac{2}{8}\) is \(\frac{-2}{8}\)
(ii) Additive inverse of \(\frac{-5}{9}\) is \(\frac{5}{9}\)
(iii) Additive inverse of \(\frac{-6}{-5}\) is \(\frac{-6}{5}\)
(iv) Additive inverse of \(\frac{2}{-9}=\left(\frac{-2}{9}\right)\) is \(\frac{2}{9}\)
(v) Additive inverse of \(\frac{19}{-6}=\left(\frac{-19}{6}\right)\) is \(\frac{19}{6}\)

Question 3.
Verify that -(-x) = x for:
(i) x = \(\frac{11}{15}\)
(ii) x = \(\frac{-13}{17}\)
Solution:

Question 4.
Find the multiplicative inverse of the following:
(i) -13
(ii) \(\frac{-13}{19}\)
(iii) \(\frac{1}{5}\)
(iv) \(\frac{-5}{8} \times \frac{-3}{7}\)
(v) \(-1 \times \frac{-2}{5}\)
(vi) -1
Solution:
(i) Multiplicative inverse of -13 is \(\frac{-1}{13}\)
(ii) Multiplicative inverse of \(\frac{-13}{19}\) is \(\frac{-19}{13}\)
(iii) Multiplicative inverse of \(\frac{1}{5}\) is 5.
(iv) \(\left(\frac{-5}{8} \times \frac{-3}{7}\right)=\frac{(-5) \times(-3)}{8 \times 7}=\frac{56}{15}\)
Multiplicative inverse of \(\frac{-5}{8} \times \frac{-3}{7}\) is \(\frac{15}{56}\)
(v) \(-1 \times \frac{-2}{5}=\frac{(-1) \times(-2)}{5}=\frac{2}{5}\)
Multiplicative inverse of \(-1 \times \frac{-2}{5}\) is \(\frac{5}{2}\)
(vi) Multiplicative inverse of -1 is -1.

Question 5.
Name the property under multiplication used in each of the following:
(i) \(\frac{-4}{5} \times 1=1 \times \frac{-4}{5}=\frac{-4}{5}\)
(ii) \(\frac{-13}{17} \times \frac{-2}{7}=\frac{-2}{7} \times \frac{-13}{17}\)
(iii) \(\frac{-19}{29} \times \frac{29}{-19}=1\)
Solution:
(i) \(\frac{-4}{5} \times 1=1 \times \frac{-4}{5}=\frac{-4}{5}\)
1 is the multiplicative identity.

(ii) \(\frac{-13}{17} \times \frac{-2}{7}=\frac{-2}{7} \times \frac{-13}{17}\)
Multiplication is commutative.

(iii) \(\frac{-19}{29} \times \frac{29}{-19}=1\)
Multiplicative inverse.

Question 6.
Multiply \(\frac{6}{13}\) by the reciprocal of \(\frac{-7}{16}\).
Solution:

Question 7.
Tell what property allows you to compute \(\frac{1}{3} \times\left(6 \times \frac{4}{3}\right) \text { as }\left(\frac{1}{3} \times 6\right) \times \frac{4}{3}\)
Solution:
\(\frac{1}{3} \times\left(6 \times \frac{4}{3}\right) \text { as }\left(\frac{1}{3} \times 6\right) \times \frac{4}{3}\)
In the given question, we use the associative property.

Question 8.
Is \(\frac{8}{9}\) the multiplicative inverse of -1\(\frac{1}{8}\)? Why or why not?
Solution:
\(\frac{8}{9} \times-\frac{9}{8}=-1\) but is not equal to 1.
So, \(\frac{8}{9}\) is not the multiplicative inverse of -1\(\frac{1}{8}\)

Question 9.
Is 0.3 the multiplicative inverse of 3\(\frac{1}{3}\)? Why or why not?
Solution:
0.3 = \(\frac{3}{10}\)
3\(\frac{1}{3}\) = \(\frac{10}{3}\)
\(\frac{3}{10} \times \frac{10}{3}=1\)
∴ Multiplicative inverse of 3\(\frac{1}{3}\) is 0.3.

Question 10.
Write:
(i) The rational number that does not have a reciprocal.
(ii) The rational numbers that are equal to their reciprocals.
(iii) The rational number that is equal to its negative.
Solution:
(i) The rational number ‘0’ does not have a reciprocal.
(ii) The rational numbers 1 and (-1) are equal to their reciprocal.
(iii) The rational number ‘0’ is equal to its negative [(0) + (0) = 0]
∴ The negative of 0 is 0.

Question 11.
Fill in the blanks:
(i) Zero has ________ reciprocal.
(ii) The numbers ________ and ________ are their own reciprocals.
(iii) The reciprocal of -5 is ________
(iv) Reciprocal of \(\frac{1}{\mathrm{x}}\) when x ≠ 0 is ________
(v) The product of two rational numbers is always a ________
(vi) The reciprocal of a positive rational number is ________
Solution:
(i) no
(ii) 1 and -1
(iii) \(\frac{-1}{5}\)
(iv) x
(v) rational number
(vi) positive

These NCERT Solutions for Class 8 Science Chapter 18 Pollution of Air and Water Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Pollution of Air and Water NCERT Solutions for Class 8 Science Chapter 18

Class 8 Science Chapter 18 Pollution of Air and Water Textbook Exercise Questions and Answers

Page 250-251

Question 1.
What are the different ways in which water gets contaminated?
Answer:
The different ways in which water gets contaminated are as follows:

• Many industries discharge harmful chemicals into rivers and streams. Examples are oil refineries, paper factories, textile and sugar mills, etc.
• The pesticides and weedicides used for the protection of crops get dissolved in water and are washed into water bodies from the fields.
• Many times, untreated sewage is thrown directly into rivers.
• Faecal matters of mammals get added into water bodies.
• Practice of bathing and washing near a water body.
• Practice of cremating the dead bodies near river banks.

Question 2.
At an individual level, how can you help reduce air pollution?
Answer:
At an individual level, one can do the following things to reduce air pollution:

• One can use vehicles consuming fuels like CNG and unleaded petrol, rather than those using fuels like diesel, petrol, etc.
• One can avoid unnecessary usage of vehicles and try using public transport more.
• One can avoid using crackers.
• One can generate awareness about air pollution among friends and neighbours.
• One can switch over to alternative fuels instead of the fossil fuels for energy requirements. These could be solar energy, hydropower and wind energy.
• One can plant trees and nurture those trees already present in the neighbourhood.
• Dry leaves can be dumped in a compost pit rather than burning in open.

Question 3.
Clear, transparent water is always fit for drinking. Comment.
Answer:
Clear, transparent water is not always fit for drinking. This is because the water which looks clean might contain disease-carrying microorganisms and dissolved impurities in it. These microorganisms and impurities could be harmful to us. Hence, it is essential to purify water before drinking.

Question 4.
You are a member of the municipal body of your town. Make a list of measures that would help your town to ensure the supply of clean water to all its residents.
Answer:
Following is a list of measures that would help a town to ensure the supply of clean water to all its residents:

• Implementation of strict laws for industrial units so that the polluted water is not disposed off directly into rivers and takes.
• Treatment of water must be done to make it free from harmful germs, chemicals and impurities.
• The water supply pipes must be properly maintained to avoid any corrosion, breakage and leakage.
• Campaigns and awareness programmes should be organised to keep water resources clean and educate people to save water.

Question 5.
Explain the differences between pure air and polluted air.
Answer:
Following are the differences between pure air and polluted air:

Question 6.
Explain circumstances leading to acid rain. How does acid rain affect us?
Answer:
Chemicals like sulphur dioxide and nitrogen dioxide are released into the air by the smoke of vehicles. These pollutants react with the water vapour present in the air and form sulphuric and nitric acid. When it rains, these acids get dissolved in rainwater making it acidic. Such kind of rain is called acid rain.

Following is the list which shows how acid rain affects us:

• It can cause skin irritation.
• It drains out nutrients like calcium from the soil.
• It causes corrosion to bridges, buildings and other metallic objects.
• It can destroy plants and aquatic life.

Question 7.
Which of the following is not a greenhouse gas?
a. Carbon dioxide
b. Sulphur dioxide
c. Methane
d. Nitrogen
Answer:
d. Nitrogen

Question 8.
Describe the ‘Greenhouse Effect’ in your own words.
Answer:
The greenhouse effect is the trapping of sunlight in earth’s atmosphere due to increased concentration of greenhouse gases like carbon dioxide, methane and water vapour in the atmosphere. These gases trap solar radiations reflected back by the earth. This helps in keeping our planet warm and thus, helps in human survival. However, an increase in the amount of greenhouse gases can lead to an increase in the earth’s temperature leading to global warming.

Question 9.
Prepare a brief speech on global warming. You have to deliver the speech in your class.
Answer:
Global warming is a serious concern of the modern times. We are using more cars and bikes than ever. We are also consuming products at a large scale which means more and more factories are coming up to satisfy our growing needs. Alt of this has resulted in an increased level of carbon dioxide in the atmosphere. The increased level of carbon dioxide has resulted in global warming. Due to this, the summers have become hotter and winters have become much colder. We are observing increased instances of flash floods. Weather changes are drastic, and this is being attributed to global warming. Global warming might result in total disaster for us. A time will come when the temperature on the earth will finish the life. It is high time we stand up and take a pledge to fight against global warming.

Question 10.
Describe the threat to the beauty of the Taj Mahal.
Answer:
Acid rain is a major threat to the beauty of the Taj Mahal. When acid rain falls on the monument made of marble, they react with marble to form a powder-like substance that is then washed away by the rain. This phenomenon is known as marble cancer. Also, the soot particles emitted from the Mathura oil refinery located near Agra are leading to the yellowing of the marble.

Question 11.
Why does the increased level of nutrients in the water affect the survival of aquatic organisms?
Answer:
The pesticides and weedicides used for the protection of crops get dissolved in water and are washed into water bodies from the fields. These act as nutrients for algae to flourish. Once these algae die, they serve as food for decomposers like bacteria. In this process, a lot of oxygen in these ponds gets used up. This results in a decrease in the oxygen level of water which kills the aquatic organisms.

NCERT Extended Learning Activities and Projects

Question 1.
In some cities, a pollution check has been made compulsory for vehicles. Visit a petrol pump in order to learn about the process of conducting a pollution check. You may systematically record your findings in the following areas:

• Average number of vehicles checked per month.
• Time taken to check each vehicle.
• Pollutants checked for.
• The process of testing
• Permissible levels of emission of various gases.
• Measures taken if the emitted gases are above the permissible limits.
• How frequently is a pollution check required?

Hint:
Do it yourself.

Question 2.
Conduct a survey in your school to investigate various environment related activities that have been undertaken. The class can divide itself into two groups, with each group looking at a different area. For example, one group can find out whether there is an environment club in the school. What are its objectives? What is its calendar of events? How can you become a member?
If your school does not have such a club, you even think of starting one along with a few of your friends.
Hint:
Do it yourself.

Question 3.
Organise a field visit to a river in or around your town with the help of your teachers. Observations followed by discussion could focus on

• the history of the river.
• cultural traditions.
• role of the river in meeting the town’s water needs.
• pollution concerns.
• sources of pollution.
• effects of pollution on the people living by the riverside as well as those living far away.

Hint:
Do it yourself.

Question 4.
Find out with the help of your teachers and the internet (if possible), whether there are any international agreements to control global warming. Which are the gases covered under these agreements?
Hint:
Following are the international agreements aimed at controlling global warming:
The Kyoto Protocol is an international agreement on climate change, developed under the United Nations Framework Convention on Climate Change (UNFCCC). The Protocol encourages 192 parties to reduce their greenhouse gas emissions, with many developed nations having binding emissions reduction targets.

The Paris Agreement was agreed in 2015 by 175 parties. The aim of the agreement is to keep global warming to ‘well below’ 2°C, and strive to limit warming to 1.5°C.

The Montreal Protocol on ozone-damaging gases does not directly address climate change, but ozone-damaging gases contribute to global warming. The 1987 Montreal Protocol binds 196 nations to reduce the emission of ozone-damaging gases, commonly used in fridges, foam and industrial applications.

Activity 1

Objective: To study the effect of various activities on the quality of air.
Procedure: Discuss in your class the impact of each of the following options on the quality of air: walking, going by bicycle, travelling by bus or other public transport, using a car individually, travelling by car pool.
Observation:

Conclusion: We can decrease the air pollution by using less polluting fuels like CNG, and using public transport or car pool.

Water Pollution: Addition of unwanted substances in water which makes water harmful for living beings and non-living things is called water pollution.

Water Pollutants: Substances which contaminate water are called water pollutants. These include harmful substances like sewage, toxic chemicals, silt, etc.

Pollution of Ganga: Ganga is one of the most significant rivers of India. It flows through many towns, cities and villages of the northern, eastern and central India. People living in these regions are dependent upon Ganga water for their livelihood as well as for their day-to-day needs. But it has been found that Ganga is one of the 10 endangered rivers of the world according to the World Wide Fund (WWF) for Nature. This is because of its extreme pollution. The river has been called dead at several places as it has no aquatic life there.

Ganga Action Plan (1985) was launched by the Government of India in order to save Ganga river. However, increasing population and industrialisation has already ruined this river beyond repair. The Government of India has launched a new initiative known as National Mission for Clean Ganga (NMCG) in 2016.

Causes of Water Pollution:

• Increasing population and industrialisation.
• Bathing, washing clothes and defecating in rivers pollute them.
• Throwing of garbage, flowers, idols of gods and polythene bags are toxic to water bodies.
• Discharge of untreated sewage in water is very harmful. Water contaminated with sewage may contain viruses, bacteria, fungi, eggs of worms and parasites which cause diseases like cholera, typhoid and jaundice.
• Industrial discharge of harmful chemicals into rivers and streams.
• Chemicals and fertilisers from agricultural fields pollute not only the rivers and lakes, but also the underground water.

Agricultural chemicals such as fertilisers, pesticides and weedicides dissolve in water in the crop fields. From there, they are washed into water bodies such as ponds, lakes, etc. These substances act as nutrients for algae, and growth of algae enhances. After some time, algae start dying due to overgrowth. Dead mass of algae is decomposed by decomposers like bacteria.

In this process, a lot of oxygen dissolved in the water gets used up. This results in a decrease in the oxygen level that may cause the death of aquatic organisms. This phenomenon of removal of all the oxygen and nutrients from a water body resulting in extinction of all life forms in it is called eutrophication.

Potable Water: Water which is fit for drinking is called potable water. A quarter of the world’s population does not have access to safe drinking water.

Methods of Water Purification:

• Municipal bodies purify water by various physical and chemical methods before supplying drinking water to the households.
• Filtration is a common method of purification of water. Various types of water filters are used in households.
• Boiling is another common method of water purification. It kills the germs in water.
• Chlorination also helps in killing the germs in water. Chlorine tablets are available for free at government hospitals.

Activity 2

Objective: To construct a simple water filter.
Materials Required: Plastic bottle, cutter, fine cloth, cotton, sand, gravel and water sample.
Procedure:

• Take a plastic bottle and cut it into two halves at the centre.
• Use the upper half of funnel by putting it upside down in the lower half.
• Make layers in it with fine cloth followed by cotton, sand and then gravel.
• Now pour dirty water through the filter and observe the filtered water.

Observation: Clean transparent water is obtained after filtration.
Conclusion: Dirty water can be purified by filtration through layers of sand.

How to Prevent Water Pollution:

• Discharge of untreated sewage and effluents in water bodies should be strictly prohibited.
• Every city should have a sewage treatment plant.
• Public awareness about harms of idol immersion in water bodies should be decreased.
• Traditional crematoriums should be replaced with electric crematoriums.

Conservation of Water: Water is a precious resource, and hence every step should be taken to conserve water. We should follow the principle of three R’s, i.e., Reduce, Reuse and Recycle, to conserve water. We should make changes in our lifestyle to prevent wastage of water. Leaking taps should be immediately repaired. Using a bucket instead of shower for a bath helps in saving water. Water left after washing clothes and vegetables should be reused for mopping and for washing cars and scooters. Sewage water should be purified so that it can be used again.

Class 8 Science Chapter 18 Pollution of Air and Water Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
What is air pollution?
Answer:
Contamination of air which makes it harmful for living beings is called air pollution.

Question 2.
How is carbon monoxide produced?
Answer:
Carbon monoxide is produced from the incomplete burning of fuels like petrol and diesel.

Question 3.
What is the full form of CFC?
Answer:
Chlorofluorocarbon.

Question 4.
Name some cleaner fuels.
Answer:
CNG, LPG, unleaded petrol, etc.

Question 5.
What is smog?
Answer:
A mixture of smoke and fog is called smog.

Question 6.
What is acid rain?
Answer:
When rainwater becomes acidic due to dissolution of the oxides of sulphur and nitrogen, it is called acid rain.

Question 7.
What is global warming?
Answer:
Overall increase in the temperature of the earth’s atmosphere is called global warming.

Question 8.
What is water pollution?
Answer:
Contamination of water which makes it harmful for living beings is called water pollution.

Question 9.
Name some water pollutants.
Answer:
Sewage, toxic chemicals, silts, etc.

Question 10.
Name some greenhouses gases.
Answer:
Methane, carbon dioxide, nitrous oxide and water vapour.

Question 11.
What is chlorination?
Answer:
Chlorination is the chemical method for purifying water, by adding chlorine tablets or bleaching powder to it.

Question 12.
What are the harmful effects of smog?
Answer:
The smog causes breathing difficulties like asthma, cough and wheezing in human beings and also reduces visibility.

Question 13.
Which gas is mainly responsible for global warming?
Answer:
CO₂ gas is mainly responsible for global warming.

Question 14.
When was Ganga Action Plain implemented?
Answer:
In 1985, Ganga Action Plan was implemented.

Question 15.
In which state is Ganga river most polluted?
Answer:
Ganga river is most polluted in Kanpur.

Question 16.
How are dust particles added in air?
Answer:
There are many sources of dust particles, e.g., stone quarries, mines, power plants, construction sites, etc.

Question 17.
How is carbon monoxide produced?
Answer:
Carbon monoxide is produced by incomplete burning of fuels petrol and diesel.

Question 18.
What are the natural causes of air pollution?
Answer:
Smoke and dust from forest fires and volcanic eruptions pollute the air and are the natural causes of air pollution.

Question 19.
How does boiling purify water?
Answer:
Boiling purifies water by killing the germs present in it.

Question 20.
What is the full form of three Rs?
Answer:
Reduce, Reuse and Recycle.

Short Answer Type Questions

Question 1.
Define CFCs. How do they create pollution?
Answer:
Electronic devices like refrigerators, air conditioners, etc., emit chlorofluorocarbons (CFCs). CFCs damage the ozone layer present in the atmosphere. Ozone layer is responsible for protecting us from harmful ultraviolet rays of the sun. These electronic devices cause depletion of ozone layer and form the ozone hole. Ultraviolet rays can cause sun bum and skin cancer. We can reduce the harmful effects of CFCs by using less harmful chemicals in place of CFCs.

Question 2.
What are the different methods to make water safe for drinking?
Answer:
Following methods may be used to make water safe for drinking:

• Filtration: It is a physical method of removing impurities from water. Many households use water purifiers that can filter water and make it safe for drinking.
• Boiling: It kills the germs present in the water. Many people boil water before drinking it.
• Chlorination: Chlorine is a natural cleaning agent for water which makes it fit for consumption. Often chlorine tablets are added in water to remove impurities from it.

Question 3.
How can we prevent water pollution?
Answer:
Water pollution can be prevented in the following ways:

• Industries should strictly adhere to the laws and do not dispose off untreated sewage water into the rivers or any other water body directly.
• More water treatment plants should be established near the industrial areas.
• We should save water as much as we can, for example, we can make sure that there are no leaking taps in the house.
• We should reuse water if we can, for example, the water that we use for washing clothes can also be used for cleaning the house floors.

Question 4.
How do automobiles pollute air?
Answer:
Automobiles usually have internal combustion engines which use petrol or diesel as fuel. Combustion of petrol or diesel results in emission of many polluting gases, tike carbon dioxide, carbon monoxide, sulphur dioxide, nitrogen dioxide, etc. Moreover, minute particles are also emitted from automobile engines. Thus, automobiles end up polluting the air.

Question 5.
Write a short note on ozone layer.
Answer:
Ozone layer is a protective layer in the atmosphere. This layer prevents the harmful ultraviolet radiations of the sun from entering our atmosphere. Ultraviolet radiations can cause skin cancer in humans and animals, and are also harmful for plants. Any damage to the ozone layer can be highly detrimental to the living beings.

Question 6.
What is eutrophication?
Answer:
Draining away of all the nutrients from a water body is called eutrophication. It happens because of excessive amount of chemicals in water. This results in proliferation of algae and other plants in water bodies with stagnant water. Once these plants die, they are consumed by decomposers. The process of decomposition utilises most of the oxygen from water. This results in deficiency of oxygen for aquatic animals. Gradually, the water body becomes devoid of any life form and is considered dead.

Question 7.
Why is it dangerous to burn polymers?
Answer:
When polymers such as plastics are burned, carbon containing gases are released. During the burning process, the carbon combines with oxygen present in the atmosphere and forms oxides of carbon. These gases have been linked with global warming, depletion of the ozone layer, acid rain, etc. They create harmful effects on our ecosystem and on the health of living beings. Hence, burning of polymers is dangerous.

Question 8.
How is our holy Ganga getting polluted? What is its effects? What is being done to prevent its pollution?
Answer:
Ganga is mainly getting polluted by dumping of untreated sewage every day. In addition, most of the human activities like bathing, washing of clothes and immersion of ashes or unburnt corpses also cause pollution. Most of the industries further contribute to the Ganga’s pollution load by releasing chemical effluents. Religious practices are also one of the major causes of Ganga’s pollution.

This pollution makes the water toxic which not only affects human health but also kills fishes in large sections of the river. To prevent pollution and to improve the quality of water of the Ganga river, government started Ganga Action Plan in 1985.

Question 9.
What measures are being taken to save Taj Mahal?
Answer:
Various measures are being taken to save the beauty of the Taj Mahal. The Supreme Court of India has ordered the industries to switch over to cleaner and pollution-free fuels. Automobiles have been ordered to use CNG and LPG as fuel. Unleaded petrol will be made available to be used as fuel in vehicles instead of leaded petrol and diesel.

Question 10.
What are the effects of global warming?
Answer:
Global warming is causing melting of icebergs and glaciers. It results in increased cases of flash floods and a rise in sea level. It also causes drastic changes in weather patterns. Severe draughts are also caused due to global warning.

Question 11.
Suggest two methods by which water can be made potable at home.
Answer:
At home, water can be made potable by killing the germs in the following ways:

1. Boiling: Boiling the water for 15-20 minutes kills all the germs.
2. Purifier with ultraviolet light: A special type of light called ultraviolet light kills germs. It is used in several water purifiers available in the market today.

Question 12.
Akshita saw a child sleeping in a car parked with closed doors and windows rolled up in an open area on a sunny day near the market. She immediately raised an alarm and with the help of police, she got the window glass rolled down.
a. Why was it not safe to keep the door with window glass rolled up for a child inside the car?
b. Name two gases that can lead to the above effect.
Answer:
a. It was not safe for the child in the car with locked doors and windows rolled up because the sunlight would result in the greenhouse effect in the car. This would increase the temperature in the car and also result in the increase in CO₂ level which would lead to suffocation.
b. Carbon dioxide gas and methane gas can lead to the greenhouse effect.

Long Answer Type Questions

Question 1.
How is water treated at the waterworks to make it potable?
Answer:
Tap water supplied to our homes is made potable in the waterworks. The processes used are as follows:
a. Sedimentation and decantation is first done to remove larger particles of solid impurities.
b. Alum is then added to the water to separate the smaller particles by making them heavier so that they settle down.
c. The water is then filtered by passing it through sand fitters.
d. Germs are killed by adding chlorine to the water. The process is called chlorination.

Question 2.
List the main pollutants of air, and give the ill effects of each.
Answer:
The main pollutants of air and their effects are as follows:
a. Suspended particulate matter (SPM): These are minute solid particles suspended in the air. The finer of these particles cannot be filtered by the hair in our nose and hence are a major cause of lung diseases.

b. Carbon monoxide: Carbon monoxide is one of the most dangerous pollutants. It is extremely poisonous. It mixes with the blood and prevents it from carrying oxygen. It may even lead to death in higher concentrations.

c. Excess carbon dioxide: Excess carbon dioxide in the atmosphere traps the heat rays of the sun. This is believed to cause an increase in the temperature of the earth. This can cause large scale melting of ice in the polar regions and also an increase in sea water. This ultimately can lead to flooding of coastal areas by sea water.

d. Sulphur dioxide and nitrogenn dioxide: Sulphur dioxide is poisonous. It affects our lungs. This gas reacts with water to form sulphuric acid, and this gives rise to acid rain. Nitrogen dioxide is also poisonous. It irritates the eyes and affects the lungs.

e. Chlorofluorocarbons (CFCs): These can damage the ozone layer in the atmosphere, causing an increase in the amount of ultraviolet rays from the sun reaching the earth. This causes damage to our eyes and skin.

f. Lead: Lead compounds cause various problems such as mental disorders and brain damage.

Question 3.
Give some methods of conserving water.
Answer:
Some of the methods of water conservation are as follows:

• Using water judiciously and avoiding wastage of water at home.
• Using better methods of irrigation.
• Recycling of water in factories and using it several times before it is disposed off.
• Treating sewage and factory wastes before disposing them off.
• Water harvesting of rainwater to raise the groundwater level.

Picture Based Question

Question 1.
Observe the given picture and answer the following questions.

a. What is the picture showing?
b. Name the government action plan to resolve this problem.
c. Give one cause of this problem.
Answer:
a. The picture is showing a polluted stretch of the river Ganga.
b. Ganga Action Plan.
c. Disposal of garbage and wastes in the river is one of the major causes of water pollution.

Question 2.
Identify the following figure and answer the questions.

a. How do such types of construction pollute air?
b. Name the possible air pollutants produced by these structures.
c. Name the gases produced by these structures causing acid rain.
Answer:
a. The factories pollute air by producing smoke, carbon particles and other gases in the air.
b. These factories produce various pollutant gases like CO, oxides of sulphur and nitrogen, etc.
c. Sulphur dioxide and nitrogen dioxide.

Question 3.
Observe the following building carefully and answer the questions:

a. Where is the building located?
b. Name the material used to construct this building.
c. Name some industries located around it.
d. Write the effect of acid rain on it.
e. What is other name of corrosion of white marble?
Answer:
a. This building is of the Taj Mahal. It is located in Agra.
b. It is made up of white marble.
c. Rubber processing factory, automobile industry, chemical industry and Mathura Oil Refinery are some industries located around the Taj Mahal.
d. The acid rain causes the corrosion of marble due to which it loses its beauty.
e. The other name of corrosion of marble is marble cancer.

These NCERT Solutions for Class 8 Science Chapter 17 Stars and The Solar System Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Stars and The Solar System NCERT Solutions for Class 8 Science Chapter 17

Class 8 Science Chapter 17 Stars and The Solar System Textbook Exercise Questions and Answers

Page 234-235

Choose the correct answer in Questions 1-3.

Question 1.
Which of the following is NOT a member of the solar system?
a. An asteroid
b. A satellite
c. A constellation
d. A comet
Answer:
c. A constellation

Question 2.
Which of the following is NOT a planet of the sun?
a. Sirius
b. Mercury
c. Saturn
d. Earth
Answer:
a. Sirius

Question 3.
Phases of the moon occur because
a. we can see only that part of the moon which reflects light towards us
b. our distance from the moon keeps changing
c. the shadow of the Earth covers only a part of moon’s surface
d. the thickness of the moon’s atmosphere is not constant
Answer:
a. we can see only that part of the moon which reflects light towards us.

Question 4.
Fill in the blanks:
a. The planet which is farthest from the Sun is ……………..
b. The planet which appears reddish in colour is ……………..
c. A group of stars that appear to form a pattern in the sky is known as a ……………..
d. A celestial body that revolves around a planet is known as ……………..
e. Shooting stars are actually not ……………..
f. Asteroids are found between the orbits of …………….. and ……………..
Answer:
a. Neptune,
b. Mars,
c. constellation,
d. satellite,
e. stars,
f. Mars and Jupiter

Question 5.
Mark the following statements as true (T) or false (F):
a. Pole star is a member of the solar system. ( )
b. Mercury is the smallest planet of the solar system. ( )
c. Uranus is the farthest planet in the solar system. ( )
d. INSAT is an artificial satellite. ( )
e. There are nine planets in the solar system. ( )
f. Constellation Orion can be seen only with a telescope. ( )
Answer:
a. F
b. T
c. F
d. T
e. F
f. F

Question 6.
Match items in column A with one or more items in column B:

Answer:
i. (e) and (g),
ii. (a),
iii. (c) and (f),
iv. (d)

Question 7.
In which part of the sky can you find Venus if it is visible as an evening star?
Answer:
The planet Venus is found in the western part of the sky after sunset and is known as an evening star.

Question 8.
Name the largest planet of the solar system.
Answer:
Jupiter is the largest planet of the solar system.

Question 9.
What is a constellation? Name any two constellations.
Answer:
A group of stars which resembles certain shape is called a constellation. Great Bear and Orion are two examples of constellation.

Question 10.
Draw sketches to show the relative positions of prominent stars in (a) Ursa Major and (b) Orion.
Answer:
(a) Ursa Major seems to be like a big ladle. There are four stars in the bowl of the ladle and three bright stars in the handle.
(b) Orion seems to be like a hunter. The three bright stars appear in the belt, while five bright stars are arranged in the form of a quadrilateral.

Question 11.
Name two objects other than planets which are members of the solar system.
Answer:
Apart from planets, asteroids and comets are also members of the solar system.

Question 12.
Explain how you can locate the Pole Star with the help of Ursa Major.
Answer:
Follow these steps for locating the pole star:

• Locate the Ursa Major and locate the quadrilateral in it.
• Select the two stars on the far side of the quadrilateral.
• Join these stars with a line and extend this imaginary line towards the top of the ladle.
• A faint star which meets this line is the Pole Star.

Question 13.
Do all the stars in the sky move? Explain.
Answer:
All the stars move but their movement is not perceptible to us. Instead of that, it is the rotation of the Earth which creates an illusion of the movement of stars across the sky. But the pole star appears to be fixed at its position. This happens because the pole star is in line with the axis of rotation of the Earth.

Question 14.
Why is the distance between stars expressed in light years? What do you understand by the statement that a star is eight light years away from the Earth?
Answer:
The interstellar distances or intergalactic distances are too big to be easily expressed in terms of kilometres. Hence, we need a more convenient unit to express them. A light year is the distance travelled by light in one year. If a star is eight light years away from the Earth, it means that the light from that star takes eight years to reach us. This also means that the star which appears to our eyes is in fact the way it looked eight years ago.

Question 15.
The radius of Jupiter is 11 times the radius of the Earth. Calculate the ratio of the volumes of Jupiter and the Earth. How many Earths can Jupiter accommodate?
Answer:
Earth and Jupiter can be considered as two spheres with radii R and R respectively. Given that the radius of Jupiter is 11 times the radius of the Earth.
Thus, R = 11 R
Volume of a sphere of radius r is given as = \(\frac {4}{3}\)πR³
Volume of the Earth = \(\frac {4}{3}\)πR³
And, volume of Jupiter =

Hence, this ratio suggests that Jupiter can accommodate 1331 Earths in it.

Question 16.
Boojho made the following sketch of the solar system. Is the sketch correct? If not, correct it.
Answer:

No, the diagram made by Boojho is not correct, because in the solar system the planets’ sequence on the basis of their distance from the Sun is like; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The mistake he had done is that he changed the positions of the planets Venus, Mars, Neptune and Uranus. Besides this, he had shown in the diagram the Asteroids belt in the gap between the orbits of Saturn and Jupiter. This is incorrect. The Asteroids belt is located between the orbits of Jupiter and Mars. The correct diagram of the solar system is as follows:

NCERT Extended Learning Activities and Projects

Question 1.
The North-South line at your place: Let us learn to draw the north-south line with the help of the shadow of a stick. Fix a straight stick vertically in the ground where the Sun can be seen for most of the day. Call the foot of the stick as point O. Sometime in the morning, mark the tip of the shadow of the stick. Say this point is A. With OA as radius draw a circle on the ground. Wait till the shadow becomes shorter and then starts increasing in size. When the shadow again touches the circle, mark it as point B. Draw the bisector of the angle AOB. This is your North-South line. To decide which side of this line shows North, use a magnetic compass.

Hint:
Do it yourself.

Question 2.
If possible, visit a planetarium. There are planetariums in many cities. In a planetarium you can see the motion of the stars, constellations and planets on a large dome.
Hint:
Do it yourself.

Question 3.
On a moonless night observe the sky for a few hours. Look out for a meteor, which appears as a streak of light. September-November is a good time for observing meteors.
Hint:
Do it yourself.

Question 4.
Learn to identify the planets visible to the naked eye and some prominent constellations such as Great Bear (Saptarshi) and Orion. Also try to locate the Pole Star and the star Sirius.
Hint:
Do it yourself.

Question 5.
Position of the rising Sun – Uttarayan and Dakshinayan:
This activity may last for several weeks. Choose a place from where eastern horizon is clearly visible. Choose also a marker, such as a tree or an electric pole, to mark the position of the rising Sun. It will be sufficient if you take the observation once every week. On any day, note down the direction of the rising Sun. Repeat this observation every week. What do you find? You would have noted that the point of sunrise changes continuously. From summer solstice (around 21 June), the point of sunrise gradually shifts towards the south. The Sun is then said to be in dakshinayan (moving south). It keeps doing so till winter solstice (around 22 December). Thereafter, the point of sunrise changes direction and starts moving towards north. The Sun is now said to be in uttarayan (moving north). From the equator, only on two days, on the days of the equinoxes (around 21 March and 23 September) the Sun rises in the east. On all other days, it rises either north of east or south of east. So, the direction of the rising Sun is not a good guide to determine directions. The Pole Star, which defines North, is a much better indicator of directions.
Hint:
Do it yourself.

Question 6.
Form a group of students. Prepare a model of the solar system showing the planets, and their relative sizes. For this take a large chart paper. Make spheres representing different planets according to their relative size. You may use newspaper, clay or plasticine to make spheres. You can cover these spheres with paper of different colours. Exhibit your models in the class.
Hint:
Do it yourself.

Question 7.
Try to make a scale model of the solar system showing distances of the planets from the Sun (Use Table 17.1 of NCERT Textbook). Did you face any difficulty? Explain it.
Hint:
Yes, the distance between the planets is so large that it becomes difficult to scale all of them to proper ratios. Moreover, there is a huge variation in the size of the planets and of the sun which becomes difficult to scale as some components may become very huge while the others become too tiny.

Question 8.
Solve the following riddle and try to make similar riddles yourself:
My first is in VAN but not in PAN
My second is in EARTH and also in HEAVEN
My third is in ONE and not in TWO
My fourth is in BUN and also in FUN
My last is in STAR but not in RADAR
I am a planet that moves round the Sun.
Hint:
Venus.

Activity 1

Objective: To show the position of the moon in its orbit and its corresponding phases. Materials Required: A big ball or a pitcher, white and black colour/paint.
Procedure:

• Take a big ball or a pitcher. Paint half of it white and half black.
• Go out into the playground with two of your friends. Draw a circle of radius of about 2 m on the ground. Divide the circle into eight equal parts as shown in the figure.

• Stand at the centre of the circle. Ask a friend to hold the ball at different points of the circle. Ask her to keep the white portion of the ball always towards the Sun. In each case, the line dividing the white and black portion is kept vertical.
• Standing at the centre of the circle, observe the visible white portion of the ball while your friend stands at the points on the circle marked earlier.
• Compare your observation with the different phases of the moon.

Observation: Following phases are observed: First quarter, waxing crescent, new moon, waning crescent, last quarter, waning gibbous, full moon, waxing gibbous. Conclusion: After the new moon day, the size of illuminated part of moon seen from Earth keeps increasing and after the full moon day it keeps on decreasing.

The Moon’s Surface: The moon’s surface is dusty, barren and has many different sizes craters on it. Moreover, there are many big and small mountains. The moon has no atmosphere and water.
Neil Armstrong was the first person to land on the moon followed by another astronaut Edwin Aldrin. Neil Armstrong landed on the moon on July 21, 1969.

Activity 2

Objective: To show that the moon completes only one rotation on its axis as it completes only one revolution around the Earth.
Procedure:

• Draw a circle of about 1 m diameter on the ground.
• Ask one of your friends to stand at the center of this circle.
• You revolve around your friend in such a manner that your face always remains towards him.
• Observe how many rotations did you complete in one revolution.

Observation: Your friend will not be able to see your back. You will complete only one rotation in one revolution.
Conclusion: The moon completes one rotation on its axis as it completes one revolution around the Earth.

The Stars: A star is a huge celestial body having its own heat and tight. Stars produce heat and light because they are full of very hot gases. The Sun is the nearest star from us. It is at a distance of about 150,000,000 (one hundred fifty million) kilometres from us. The next nearest star from us is Alpha Centauri. This is about 40,000,000,000,000 (forty trillion) kilometres from us.

Light Year: The interstellar distances or intergalactic distances are too big to be easily expressed in terms of kilometres. Hence, we need a more convenient unit to express them. A light year is the distance travelled by light in one year. Distances of stars are usually expressed in terms of light years. The sunlight takes about 8 minutes to travel from the Sun to the Earth. This implies that the Sun is at a distance of 8 light minutes from us. Alpha Centauri is 4.3 light years away from us.

Pole Star: The pole star also called Polaris is situated close to the Earth’s axis of rotation. As a result, the position of the pole star appears to be fixed. This is unlike the other stars as their position keeps on changing with respect to the Earth.

Activity 3

Objective: To show that the pole star lies close to the axis of rotation of the Earth.
Materials Required: An umbrella and a white paper.
Procedure:

• Take an umbrella and open it.
• Make about 10-15 stars out of white paper.
• Paste one star at the position of the central rod of the umbrella and others at different places on the cloth near the end of each spike.
• Now, rotate the umbrella by rotating its central rod in your hand.
• Observe the stars on the umbrella. Observe if there is any star which does not appear to move.

Observation: The star situated at the central rod of the umbrella does not appear to move.

Conclusion: Pole star is the star which is situated in the direction of the Earth’s axis. It does not appear to move.

Constellation: The stars forming a group that has a recognisable shape is called a constellation. These constellations appear to move from east to west. The shape of all constellations seen in the sky always remains the same. Each constellation has been given the name of an animal or object, which it resembles.

Major Constellations
i. Great Bear: One of the most famous constellations which you can see during summer time, in the early part of the night is Ursa Major. It is also known as the Big Dipper, the Great Bear or the Saptarshi. There are seven prominent stars in this constellation. It appears like a big ladle or a question mark. There are three stars in the handle of the ladle and four in its bowl. Ursa Major revolves around the pole star.
ii. Orion: Orion is also called the Hunter or Kalpurush. The three prominent stars in this constellation mark the belt of the hunter. An elaborate structure shows a man with a club and a shield. This constellation is visible in the southern sky, and this can be easily seen during summer months in India.

Sirius: The Sirius star is the brightest star of the sky. It can be located with the help of the Orion constellation. If we draw an imaginary line through the three stars in the middle of the Orion and extend further, we will reach the brightest star, Sirius.

iii. Cassiopeia: It is a constellation found in the Northern hemisphere and can be located in the sky during the winter season at the beginning of the night. The shape of this constellation resembles a W or a distorted M.

iv. Leo Major: It contains several bright stars making it one of the most easily recognisable constellations in the night sky. It is visible in both the northern and the southern hemispheres. In northern hemisphere, it can be seen from January to June.

The Solar System: The Sun and the celestial bodies which revolve around it forms the solar system. It consists of a large number of bodies such as the sun, planets, comets, asteroids and meteors.

The Sun: The Sun is the nearest star amongst all the stars present in the universe. It is the main source of heat and tight for all the planets specially Earth.

The Planets: Our solar system comprises eight planets namely Mercury, Venus, Earth, Mars, Jupiter, Saturn. Uranus and Neptune. The planets are like stars, but they do not have light of their own. They merely reflect the sunlight that falls on them. The planets keep changing their positions with respect to the stars. These are the objects revolving around the Sun in a fixed path. This fixed path is termed as an orbit.

Period of Revolution: It is the time taken by a planet to complete full revolution around the Sun.
Period of Rotation: It is the time taken by planet to complete one rotation around its own axis.

Activity 4

Objective: To show that planets move in their own orbits and they do not collide with one another.
Procedure:

• Go out into the playground with four or five of your friends.
• Draw fourcirclesofradii 1m, 1.8 m, 2.5 m and 3.8 m, all having a common centre.
• Ask one of your friends to stand in the center and represent the Sun. Your other four friends may represent Mercury, Venus, Earth and Mars.
• Ask your friends to move around the Sun in anti-clockwise direction in their own orbits. Do they collide with one another?

Observation: They do not collide with one another, because they have fixed orbits of their own.

Conclusion: Planets moves in their own orbits so they do not collide.

Planets Revolving Around the Sun:
i. Mercury: It is the nearest planet to the Sun and the smallest planet present in our solar system. It is very difficult to observe it, as most of the time it is hidden in the glare of the Sun. It has no satellite of its own.

ii. Venus: It is the brightest planet. It is also called a morning or an evening star. It has no natural sateLlite of its own. Venus rotates from east to west, unlike the Earth which rotates from west to east. Venus also shows phases just like the moon.

iii. Earth: Earth is the only planet where life is known to exist. The Earth appears blue-green due to the reflection of light from water and landmass on its surface. The axis of rotation of the Earth is not perpendicular to the plane of its orbit. The tilt is responsible for the change of seasons on the Earth. It has only one satellite, the moon, of its own.

iv. Mars: It is called the red planet because it has a reddish colour. It has two moons of its own. Indian Space Research Organisation (ISRO) launched India’s first Mars orbiter mission – Mangalyaan on November 5, 2013.

v. Jupiter: It is the largest planet of the solar system. It rotates very rapidly on its axis. Jupiter has a large number of satellites, and also has faint rings around it.

vi. Saturn: Saturn has thick rings around it that cannot be easily viewed with naked eyes. It has 30 natural satellites of its own. The density of Saturn is less than that of water making it the least dense planet of the solar system.

vii. Uranus: Just like Venus, Uranus rotates from east to west. It has 21 natural satellites. The rotation axis of Uranus is highly tilted. The atmosphere of Uranus mainly comprises hydrogen and methane gases.

viii. Neptune: It is the farthest planet from the Sun, hence, it takes the largest time to revolve around the Sun. It has 8 natural satellites of its own.

Satellites: These are the celestial bodies which orbit around other celestial body. Example: The moon is a satellite of the Earth.

Artificial satellites: These are the man-made satellites revolving around the Earth. These are launched from the Earth and revolve much closer to the Earth than its natural satellite, the moon. Aryabhatta was the first Indian satellite. Other examples are INSAT, IRS, Kalpana-1, etc. They are used for weather forecasting, transmitting radio
and television signals, telecommunication and remote sensing.

Asteroids: The smalt rock-like substances that are orbiting around the Sun are called asteroids. These are found in large gap between the orbits of Mars and Jupiter and form the asteroid belt. They are visible only through large telescopes.

Comets: Comets appear as a bright head with a long tail. They revolve around the Sun in highly elliptical orbits. The tail of a comet is always directed away from the Sun. The length of the tail grows in size as it approaches the sun.

Meteors: These are small objects that sometimes enter the Earth’s atmosphere. These occasionally enter the Earth’s atmosphere with very high speed. Due to friction of Earth’s atmosphere, these glow and then evaporate quickly. That is why the bright steak lasts for a very short time. However, meteors are large and so they can reach the Earth before they evaporate completely. Meteorites are the bodies that reach the Earth. Meteorites help scientists in investigating the nature of the material, from which the solar system was formed.

Class 8 Science Chapter 17 Stars and The Solar System Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
What do you mean by celestial objects?
Answer:
Any naturally occurring object in the observable universe is called a celestial object. Stars, planets, satellites, comets, etc., are examples of celestial objects.

Question 2.
What is a light year?
Answer:
The distance travelled by light in one year is called a light year.

Question 3.
What is the solar system?
Answer:
The Sun, the eight planets, their moons and the other celestial objects that revolve around the Sun, form the solar system.

Question 4.
Why can’t we see the stars during the day?
Answer:
Due to the brightness of the Sun during the day, we cannot see the stars at that time.

Question 5.
If the planet Saturn is immersed in water, will it float or sink? Why?
Answer:
The planet Saturn will float because its density is less than that of water.

Question 6.
Define planets.
Answer:
Planets are heavenly bodies orbiting around the Sun. They look like stars but they do not have their own tight.

Question 7.
What is the direction of rotation of the Earth on its axis?
Answer:
West to east.

Question 8.
What is revolution?
Answer:
The movement of a celestial body in its orbit is called revolution.

Question 9.
What is rotation?
Answer:
The movement of a celestial body around its own axis is called rotation.

Question 10.
What is a satellite?
Answer:
A celestial body which moves around a planet is called a satellite.

Question 11.
Name the first Indian artificial satellite.
Answer:
Aryabhatta.

Question 12.
What are comets?
Answer:
A comet is a celestial body which revolves around the Sun in a highly elliptical path.

Question 13.
After how many years does Halley’s Comet appear from the Earth?
Answer:
76 years.

Question 14.
What is universe?
Answer:
The limitless expanse of space around us consisting of our solar system and the other stars and galaxies is called universe.

Question 15.
What are stars?
Answer:
The celestial bodies which have their own heat and light are called stars.

Question 16.
Name a star which lies stationary in the sky.
Answer:
Pole star.

Question 17.
Name the natural satellite of the Earth.
Answer:
Moon.

Question 18.
What is the distance of the Sun from the Earth? What is this distance called?
Answer:
The Sun is nearly 150,000,00 km away from Earth. The distance of Sun from Earth is 8.3 tight minutes and is called Astronomical Unit (AU).

Question 19.
Name the star which is nearest to the Earth after the Sun.
Answer:
Alpha Centauri.

Question 20.
Name some artificial satellites.
Answer:
INSAT, IRS, EDUSAT, Kalpana-1, etc.

Short Answer Type Questions

Question 1.
Write a short note on Pole Star.
Answer:
While other stars appear to move with gradual progress of night, the pole star remains fixed at its position. This happens because the pole star is in line with the axis of the Earth. The pole star appears in the northern sky. It is only visible from the northern hemisphere. However, the fixed position of the pole star has helped humans since ages.

Question 2.
What are asteroids?
Answer:
Asteroids are made up of rocks, metallic minerals and some ice. They are remnants of formation of the solar system. The asteroid belt is present between the orbits of Mars and Jupiter. This belt extends between 2.3 and 3.3 AU from the Sun.

Question 3.
Why is the Venus called the morning or the evening star?
Answer:
Venus is the brightest planet in the night sky. Venus is visible in early evening during the summer months and in early morning during winters. Due to this, Venus is also called the morning or evening star.

Question 4.
Why is the distance between the stars expressed in light years?
Answer:
Stars are very far from the Earth. Their distance, if written in kilometres, will not be convenient to read and memorise. Therefore, large distances are expressed in light year. It is the distance travelled by light in one year.

Question 5.
Why is Venus the hottest planet even though Mercury is closest to the Sun?
Answer:
Venus is a very hot planet (about 480°C), even hotter than Mercury, though it is further away from the Sun. This is because of the high percentage of carbon dioxide in its atmosphere, which causes heating due to the greenhouse effect.

Question 6.
How are stars different from planets?
Answer:

Question 7.
Differentiate between natural and artificial satellites.
Answer:
Natural satellites:

• Natural satellites are far away from the planets.
• They are comparatively larger in size and are present naturally.
• They do not have light of their own. They reflect the light from the Sun.

Artificial satellites:

• Artificial satellites are man-made and are sent from the Earth with specific requirements.
• Their distance, size and mass are lesser than the natural satellites.
• The electrical power required by artificial satellites is provided by panels of solar cells and small nuclear reactors.

Question 8.
Explain in brief about comets.
Answer:
Comets are small bodies which revolve around the Sun in highly elliptical orbits. They become visible from the Earth only when they come closer to the Sun. They are characterised by a small head followed by a long tail.

Question 9.
Why was Pluto out as a planet?
Answer:
In 2006, the international astronomical union adopted a new definition of a planet. According to this definition, a full fledged planet is an object that orbits the Sun and is large enough to have become round due to its own gravity.

In addition, the planet has to dominate the neighbourhood around its orbit. Pluto does not dominate its orbit as Charon, its large moon, is only half the size of Pluto, whereas all the true planets are far larger than their moons. Also, planets sweep up asteroids, comets, and other debris along their orbits while Pluto’s orbit is somewhat untidy. Hence, Pluto does not fit in the new definition of a planet and is no longer counted as a planet.

Question 10.
Why are meteors commonly called shooting stars?
Answer:
Meteors are smalt objects mainly made up of stones present in space. They regularly enter the Earth’s atmosphere. When a meteor enters the Earth’s atmosphere at high speed, it is heated by friction with air, and in most of the cases it burns to ashes in a very short time. As it heats up and burns, it appears as a glowing streak of light and is called a shooting star. Though it is called a shooting star, it is not a star at all.

Question 11.
What major factors have enabled life to evolve and survive on the Earth?
Answer:
All the factors necessary for life are present only on Earth. These are as follows:

• It has an atmosphere containing gases, like oxygen and carbon dioxide, which are necessary for survival.
• It has water in liquid form, which is essential for life.
• Being at the right distance from the Sun, it is not as hot as Mercury or Venus, nor is it as cold as the planets which are further away. This maintains a suitable temperature to support life.

Question 12.
Give five ways in which artificial satellites are useful to us.
Answer:
Artificial satellites are very useful to us in the following ways:

1. They help in television and radio transmission.
2. They help in telephone communication.
3. They help us to study and forecast the weather by sending cloud pictures taken from the space to the Earth.
4. They help in locating minerals.
5. They help in studying agricultural yield on the Earth by photographing from above.

Long Answer Type Questions

Question 1.
Explain some salient features of the Earth.
Answer:
Earth is the only planet where life is known to exist. According to scientists, earth is at a perfect distance from the sun and hence it has the right combination of gases, materials and temperature to support life. A major portion of the Earth’s surface is covered with water, and hence it appears blue from the space. Due to this, Earth is also called the blue planet.

Earth’s orbital plane is tilted at 23.5° on its equatorial plane. This means that Earth’s axis is tilted at 66.5° on its orbital plane. Seasons change on Earth because of this tilt.

Question 2.
Explain some common constellations in the night sky.
Answer:
A group of stars having a recognisable shape is called constellation. Constellation is an internationally defined area of celestial sphere. Some examples of constellations are described below:
a. Ursa Major can be seen in the sky during summer time in the early part of the night. It is also known as Big Dipper or Saptarshi. There are seven prominent stars in this constellation. It appears like a big question mark or ladle.
b. Orion can be seen in the sky during winter in the late evenings. It has seven or eight stars. The brightest star Sirius is located close to the Orion.
c. Cassiopeia is a constellation that can be seen in the northern sky. It is visible during winter in the early part of night. It looks like distorted letter W or M.

Question 3.
What are planets? Describe all the planets in the solar system in brief.
Answer:
The bodies which revolve around the Sun in a certain orbit are called planets. There are eight planets in our solar system which are as follows:

• Mercury: It is the nearest planet to the Sun and is the smallest planet of our solar system. It is very close to the Sun so it is quite difficult to observe it. It has no satellite of its own.
• Venus: It is the Earth’s nearest planetary neighbour. It is the brightest planet in the night sky. Sometimes, it appears in the eastern sky before the sunrise, and sometimes in western sky just after the sunset. Thus, it is also called morning or evening star. It has no satellite of its own.
• Earth: It is the third planet. The Earth is the only planet in the solar system, on which life exists. Earth appears blue-green due to the reflection of light from water and landmass. It has only one moon.
• Mars: The fourth planet is called Mars. It is called the red planet. Mars has two small satellites.
• Jupiter: It is the largest planet of the solar system. It rotates very rapidly on its axis and has a large number of satellites. It also has a faint ring around it.
• Saturn: Saturn lies beyond Jupiter. It contains beautiful rings around it which are not visible with the naked eye. It has a large number of satellites. It is least dense among all planets; in fact its density is less than that of water.
• Uranus: It can be seen via telescope only. In its orbital motion, it appears to roll on its side. It rotates from west to east.
• Neptune: It can also be seen via telescope only. Neptune has a planetary ring system, though one much less substantial than that of Saturn.

Question 4.
What do you understand by phases of moon? What is the reason for the different phases of moon? Explain with the help of a diagram.
Answer:
The moon does not appear similar on all days, rather its shape changes every day. The different shapes of the moon are called the phases of moon.

Reason of Phases of Moon: The moon takes same time in completing a revolution and a rotation. Due to this, only one surface of the moon is visible from the Earth. The surface which is visible from the Earth gets varying ratios of light and shadow. Due to this, we are able to see different phases of the moon.

Picture-Based Questions

Question 1.
a. Draw a diagram to show the asteroid belt.
b. Name the planets between which the asteroid belt occurs.
Answer:
a.

b. Asteroid belt occurs between Mars and Jupiter.

Question 2.
Draw a diagram to show that moon is visible due to reflected sunlight, and answer the following questions:
a. How many moons does the Earth have?
b. Name a planet which has no moon.
Answer:

a. There is only one moon of the Earth.
b. Mercury is a planet which has no moon.

These NCERT Solutions for Class 8 Science Chapter 16 Light Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Light NCERT Solutions for Class 8 Science Chapter 16

Class 8 Science Chapter 16 Light Textbook Exercise Questions and Answers

Page 212-213

Question 1.
Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Answer:
It is not possible to see in a dark room because of absence of light. We know that we are able to see something when light falls on it. If there is light outside the room, the objects outside the room can be seen.

Question 2.
Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Answer:
Difference between regular and diffused reflection:

Diffused reflection is not due to the failure of the laws of reflection. It is caused by the irregularities in the reflecting surface.

Question 3.
Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.
a. Polished wooden table
b. Chalk powder
c. Cardboard surface
d. Marble floor with water spread over it
e. Mirror
f. Piece of paper
Answer:
a. Regular reflection will occur because it is a smooth surface.
b. Diffused reflection will take place because it is a rough surface
c. Diffused reflection will take place because it is a rough surface having irregularities.
d. Diffused reflection will take place because marble and water will form prism.
e. Regular reflection will take place because plane mirror is a smooth surface.
f. Diffused reflection will take place because paper is a rough surface.

Question 4.
State the laws of reflection.
Answer:
The laws of reflection are as follows:

• First Law of Reflection: The incident ray, the reflecting ray and the normal at the point of incidence, all lie in the same plane.
• Second Law of Reflection: Angle of incidence is always equal to the angle of reflection.

Question 5.
Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
Answer:
For this, take a chart paper, a torch and a plain mirror. Paste a black paper in the front of the torch and make a small hole in the centre of the black paper. This will give you a thin beam of light.

a. Keep the chart paper on a table in such a way that some portion of the chart paper is outside the edge of the table. Keep the mirror vertically on the chart paper.

b. In a dark room, direct the beam of light on the mirror so that the incident ray is along the surface of the chart paper. Mark the incident ray, the normal and the reflected ray.

c. After that, fold the portion of the chart paper which is jutting out of the table. Repeat the above step and observe the incident ray and the reflected ray.

d. It is observed that the reflected ray cannot be seen on the folded portion of the chart paper. The folded portion of the chart paper is in a different plane than the remaining part of the chart paper. This proves that the incident ray, the normal and the reflected ray, all lie in the same plane.


The incident ray, the reflected ray and the normal at the point of incidence lie in the same plane

Question 6.
Fill in the blanks in the following:
a. A person 1 m in front of a plane mirror seems to be …………….. m away from his image.
b. If you touch your …………….. ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with ……………..
c. The size of the pupil becomes …………….. when you see in dim light.
d. Night birds have …………….. cones than rods in their eyes.
Answer:
a. 2,
b. left left hand,
c. large,
d. less

Choose the correct option in Questions 7-8.

Question 7.
Angle of incidence is equal to the angle of reflection.
a. Always
b. Sometimes
c. Under special conditions
d. Never
Answer:
a. Always

Question 8.
Image formed by a plane mirror is
a. virtual, behind the mirror and enlarged
b. virtual, behind the mirror and of the same size as the object
c. real at the surface of the mirror and enlarged
d. real, behind the mirror and of the same size as the object.
Answer:
b. virtual, behind the mirror and of the same size as the object

Question 9.
Describe the construction of a kaleidoscope.
Answer:
A kaleidoscope is composed of a hollow tube. Three mirrors are placed in the form of a triangular tube with their reflecting surfaces facing each other. One end of the tube is covered with a transparent sheet and another end is covered with an opaque sheet. There is an eyehole in the opaque sheet. Bits of glass are filled inside the tube. When a kaleidoscope is turned, we get to see various patterns in it. These patterns are formed because of multiple reflections.

Question 10.
Draw a labelled sketch of the human eye.
Answer:

Question 11.
Gurmit wanted to perform Activity 16.8 (of NCERT textbook) using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advice?
Answer:
Laser light is harmful for the human eye, because its intensity is very high. It can cause damage to the retina and lead to blindness. Hence, it is advisable not to look at a laser beam directly.

Question 12.
Explain how you can take care of your eyes.
Answer:
Some tips for care of eyes are as follows:

• Do not read in too bright or too dim light.
• Do not look directly at a bright object or at the sun.
• Do not keep the book too close to your eyes, while reading. Do not keep the book too far either.
• If something gets into the eye, do not rub the eye. Wash it with cold water.
• In case of any problem, like itching or burning sensation, consult an ophthalmologist. A doctor who specialises in the diseases of eyes is called an ophthalmologist.

Question 13.
What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Answer:

Given: ∠i + ∠r = 90° …….. (i)
We know that, ∠i = ∠r (Law of reflection)
Replacing ∠r in equation (i) with ∠i, we get
∠i + ∠i = 90°
or 2 ∠i = 90°
or ∠i = \(\frac{90^{\circ}}{2}\)
or ∠i = 45°.

Question 14.
How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Answer:
When the mirrors are parallel to each other, we get infinite number of images.

Question 15.
Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in figure. Draw the reflected ray from the second mirror.

Answer:
The laws of reflection are used to obtain the path of reflected light.

It can be observed that the given ray of light will reflect from the second mirror at an angle of 60°.

Question 16.
Boojho stands at A just on the side of a plane mirror as shown in the given figure. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?

Answer:
A plane mirror forms a virtual image behind the mirror. The image is as far behind the mirror as the object is in front of it. A cannot see his image because the length of the mirror is too short on his side. However, he can see the images of objects placed at points P and Q, but cannot see the image of the object placed at point R (as shown in the given figure).

Question 17.
a. Find out the position of the image of an object situated at A in the plane mirror (see figure).

b. Can Paheli at B see this image?
c. Can Boojho at C see this image?
d. When Paheli moves from B to C, where does the image of A move?
Answer:
a. Image of the object placed at A is formed behind the mirror. The distance of the image from the mirror is equal to the distance of A from the mirror. Image of A is shown in the given figure.

b. Yes. Paheli at B can see this image.
c. Yes. Boojho at C can see this image.
d. Image of the object at A will not move. It will remain at the same position when Paheli moves from B to C.

NCERT Extended Learning Activities and Projects

Question 1.
Make your own mirror. Take a glass strip or glass slab. Clean it and put it on a white sheet of paper. See yourself in the glass. Next put the glass slab on a black sheet of paper. Again look into the glass. In which case you see yourself better and why?
Hint:
We can see our image clearly when the black paper is kept below the glass slab. When we put the glass slab on a white or coloured paper, the light which reflects, reaches our eyes from all directions. However, if we keep a black sheet below the glass slab, the sheet will absorb all the light rays, but our image formed will reflect back to our eyes clearly (since other rays will be absorbed by the black paper).

Question 2.
Make friends with some visually impaired students. Enquire from them how they read and write. Also find out how they are able to recognise objects, hurdles and currency notes.
Hint:
Do it yourself.

Question 3.
Meet an eye specialist. Get your eyesight checked and discuss how to take care of your eyes.
Hint:
Do it yourself.

Question 4.
Survey your neighbourhood. Find out how many children below the age of 12 years use spectacles. Find out from their parents what, in their view, could be the reason for the weak eyesight of their children.
Hint:
Some common reasons for poor eyesight include lack of sufficient nutrients in diet, e.g., lack of vitamin A, lack of exercise, spending large time on screens like television, computer, cell phone, etc. Unhealthy lifestyle is one of the most common causes of poor eyesight.

Activity 1

Objective: To make a kaleidoscope.
Materials Required: Three rectangular mirror strips (15 cm × 4 cm), cardboard, circular glass plate, broken pieces of glass and coloured beads.
Procedure:

• Take three rectangular mirror strips each about 5 cm long and 4 cm wide.
• Join them as a prism and place inside a circular cardboard tube or tube of a thick chart paper. Make sure that the tube is slightly longer than the mirror strips.
• Close one end of the tube by a cardboard disc having a hole in the center, through which you can see. To make the disc durable, paste a piece of transparent plastic sheet under the cardboard disc.
• At the other end, touching the mirror, fix a circular plane glass plate.
• Place on this glass plate several small pieces of coloured glass, broken pieces of coloured bangles, etc.
• Close this end of the tube by a ground glass plate.
• Allow enough space for the colour pieces to move around.
• Your kaleidoscope is ready. When you peep through the hole, you will be able to see a variety of patterns in the tube.

Dispersion: Sunlight or white light is made up of seven colours. Splitting of white light into its constituent colours is called dispersion of light.

Activity 2

Objective: To show the dispersion of white light.
Materials Required: A plane mirror, a bowl and water.
Procedure:

• Take a plane mirror of a suitable size and place it in a bowl.
• Fill the bowl with water.
• Put this arrangement near a window in such a way that direct sunlight falls on the mirror.
• Adjust the position of the bowl so that the reflected light from the mirror falls on a nearby wall. See what happens.

Observation: Reflected light will be seen to have many colours. As the mirror and water together form a prism, it breaks up the light into its constituent seven colours on the wall.

Conclusion: Sunlight is white light. It consists of seven colours and splits into its constituent colours when it is passed through a prism. This is known as dispersion of light.

Human Eye: Eyes are the sense organs which give us the sense of vision. The human eye is almost spherical in shape. Following are the main structures in the human eye:

i. Cornea: The eyeball is covered with a tough layer. This layer is transparent on the front. This transparent portion is called cornea.

ii. Iris: Iris is a thin circular structure. It works like the shutter of a camera. It controls the amount of light entering the eye. The colour of iris imparts distinct colour to the eyes of an individual. A person with blue iris has blue eyes.

iii. Pupil: There is a hole in the centre of iris. This is called pupil. Light enters the eye through the pupil. When the light is bright, iris contracts reducing the size of pupil and thus allows less light into the eye. When the light is dim, iris dilates increasing the size of pupil and thus allows more light into the eye.

iv. Lens: Lens is present behind the pupil. The lens in the human eye is a convex lens. The size and thickness of the lens change as per the distance of the object to be viewed.

v. Retina: Retina is at the back of the eye and makes the inner layer of the eyeball. Images are formed on the retina and thus it works like a screen. There are photosensitive cells in the retina. These cells are of two types:

• Cone cells: The cone cells are sensitive to bright light. They also give the sense of colour.
• Rod cells: The rod cells are sensitive to dim light. They do not give the sense of colour.

vi. Optic Nerve: The optic nerve emerges from the back of the eyeball. This nerve goes to the brain and sends messages to the brain to interpret the image.

vii. Blind Spot: The junction of retina and the optic nerve is called the blind spot. There is no photosensitive cell at this spot, and hence no image is formed at this spot of the retina. A yellow spot is located at the centre of the retina. It has maximum concentration of light sensitive cells.

Activity 3

Objective: To recognise the blind spot. Materials Required: A sheet of paper, a pen or a sketch pen.
Procedure:

• Make a round mark and a cross on a sheet of paper with the spot to the right of the cross. The distance between two marks should be 6-8 cm.
• Hold the sheet of paper at arm’s length from the eye.
• Close your left eye. Look continuously at the cross.
• Move the sheet slowly towards you keeping your eye on the cross, and observe.
• Now, close your right eye. Look at the round mark now and repeat the activity.

Observation: In the first case, the round mark disappears while in second case, the cross disappears.

Conclusion: The disappearance of the cross or the round mark shows that there is a point on the retina which cannot send messages to the brain when light falls on it.

Persistence of Vision: An image stays on the retina for about 1/16^th of a second. This feature is called persistence of vision. Due to this, when many still images are shown in a sequence at a high speed they give the illusion of moving images. Movies and animations are made by exploiting this property of the human eye.

Activity 4

Objective: To show that the impression of an image persists for about 1/16^th of a second on the retina even after we have stopped seeing the object, sketch pens, etc.
Materials Required: A piece of cardboard, a string, sketch pens, etc.
Procedure:

• Take a square piece of cardboard of size 6-8 cm.
• Make two holes as shown in figure.
• Thread a string through the two holes.
• Draw/paste a cage on one side of the cardboard and a bird on the other side.
• Twist the string and make the card swirl rapidly. Observe what happens.

Observation: On doing so, the bird is seen in the cage.

Conclusion: The image of bird persists for 1/16^th of a second. The image of cage is formed on the retina in a time less than 1/16^th a second. Hence, both the images persist together on the retina and the bird is seen inside the cage. This happens due to persistence of vision.

Range of Normal Vision: The distance between 25 cm and infinity is called the range of normal vision. The most comfortable distance at which one can read with a normal eye is about 25 cm.

Cataract: During old age, the crystalline lens becomes milky and cloudy in a cataract patient and thus the person cannot see clearly. Cataract is treated by surgery. In cataract surgery, the cloudy lens is removed and an artificial lens is transplanted in the eye.

Night Blindness: Night blindness, also called nyctalopia, is a defect of vision in which people cannot view things properly in dim light or at night. It mainly occurs due to the lack of vitamin A in our body. Hence, we should always take meals that are rich in vitamin A such as broccoli, green vegetables, carrot, milk, eggs, curd, papaya, mango, etc.

Non-optical and Optical Aids for Visually Impaired:
Non-optical aids:

• Visual aids: Magnify words, give suitable light intensity.
• Tactual aids: Braille writer slate and stylus.
• Auditory aids: Cassettes, tape recorders, talking books, etc.
• Electronic aids: Talking calculators and computers, CCTV (closed circuit television), etc.

Optical aids: Bifocal lenses, contact lenses, tinted lenses, magnifiers and telescopic aids.

Louis Braille: Louis Braille was bom in 1809 in France. He became blind during early childhood because of an accident. He developed the Braille script in 1824.

Braille Script: The Braille scripts are written in the form of raised dots. This system is composed of 63 characters. Each character is written on a grid of 6 ceils. This can be written with the help of Braille slate and stylus. A reader needs to touch the Braille script to read it.

Care of Eyes: Eyes are very important organs and they are sensitive too. So, proper care of eyes is very important. Some tips for care of eyes are as follows:

• Do not read in too bright or too dim light.
• Do not look directly at a bright object or at the sun.
• Do not keep the book too close to your eyes, while reading. Do not keep the book too far either.
• If something gets into the eye, do not rub the eye. Wash it with cold water.
• In case of any problem, like itching or burning sensation, consult an ophthalmologist. A doctor who specialises in the disease of eyes is called an ophthalmologist.

Class 8 Science Chapter 16 Light Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
What is light?
Answer:
Light is a form of energy which gives the sensation of vision.

Question 2.
What is reflection of light?
Answer:
When light falls on a shiny surface, most of it bounces back. This phenomenon of bouncing back of light after striking a surface is called reflection of light.

Question 3.
What is incident ray?
Answer:
The ray of light falling on a reflecting surface is called incident ray.

Question 4.
What is reflected ray?
Answer:
The ray of light coming back from a reflecting surface after reflection is called reflected ray.

Question 5.
What is angle of reflection?
Answer:
The angle between the reflected ray and the normal is called angle of reflection.

Question 6.
What is persistence of vision?
Answer:
An image stays on retina for 1/16^th of a second. This is called persistence of vision.

Question 7.
Which phenomenon is responsible for the creation of rainbows?
Answer:
Dispersion of light.

Question 8.
At which part of the eye there is no sense of vision?
Answer:
Blind spot.

Question 9.
Name the cells of retina which are responsible for vision during daytime.
Answer:
Cones.

Question 10.
What type of reflection forms an image?
Answer:
Regular reflection forms an image.

Question 11.
Who invented the Braille system?
Answer:
Louis Braille, a visually challenged person, developed the Braille system.

Question 12.
What is the use of kaleidoscope?
Answer:
Designers of wallpapers, fabrics, artists use kaleidoscope to get ideas for new patterns.

Question 13.
What are the common defects of vision?
Answer:
Myopia, hypermetropia, cataract and presbyopia.

Question 14.
Name the small opening in the iris.
Answer:
Pupil.

Short Answer Type Questions

Question 1.
How does light enable us to see something?
Answer:
When light falls on an object, some of the light is reflected back to our eyes. Thus, we are able to see an object because of light being reflected from the object.

Question 2.
Describe the dispersion of light by a prism.
Answer:
When light falls on one of the faces of the prism, it gets split up into seven colours while emerging out of the other face of the prism. This band of colours is called spectrum and the phenomenon is called dispersion of light.

Question 3.
Why in old ages does the eyesight become foggy? How can this defect be removed?
Answer:
Sometimes, particularly in old age, eyesight becomes foggy. It is due to the eye lens becoming milky and cloudy. When it happens, persons are said to have cataract. There is a loss of vision, sometimes extremely severe. It is possible to treat this defect. The opaque lens is removed and a new artificial lens is inserted. Modem technology has made this procedure simpler and safer.

Question 4.
Do we see all objects due to reflected light?
Answer:
Nearly everything around us is visible due to reflected light. Moon, for example, receives light from the sun and reflects it. That is why we see the moon. These objects are called illuminated objects. There are other objects which give their own light such as the sun, fire, flame of candle, electric lamp and others. These are known as luminous objects. Hence, only illuminated objects are seen due to reflected light.

Question 5.
Movies that we see are a collection of separate pictures that we see as moving. How?
Answer:
The impression of an image does not vanish immediately from the retina. It persists there for about 1/16th of the second. So, if the images flash on the eye at a rate faster than 16 images per second, the eye perceives a moving picture of the objects. The movies that we see are actually separate pictures. They are made to move across the screen usually at the rate of 24 per second (faster than 16 per second).

Question 6.
How many kinds of cells are found in the retina of the eye? What are their functions?
Answer:
There are two kinds of cells in retina:

• Cones which are sensitive to bright light and give a sense of colour.
• Rods which are sensitive to dim light.

Question 7.
Explain the function of the muscles attached to the lens of the eye.
Answer:
In order to see the nearby objects, muscles attached to the lens contract and the lens becomes thicker. On the other hand, muscles relax and the lens becomes thinner when distant objects are to be seen. This changing of the thickness of the eye lens is called accommodation.

Question 8.
Why should one include the vitamin A rich eatables in their diet? What are the main sources of vitamin A?
Answer:
Lack of vitamin A in foodstuff is responsible for many eye troubles. Most common amongst them is night blindness. One should, therefore, include the components which have vitamin A in their diet. Raw carrots, broccoli and green vegetables such as spinach (palak), methi, amaranth and cod liver oil are rich in vitamin A. Eggs, milk, curd, paneer, butter, ghee and fruits such as papaya, banana, mango, apple, dates, etc., also contain plenty of vitamin A.

Question 9.
What are the non-optical and optical aids for visually challenged persons?
Answer:
There are many resources which enable the visually challenged persons to develop their capabilities. These resources can be of two types: non-optical aids and optical aids.
Non-optical aids:

• Visual aids: Magnify words, give suitable light intensity.
• Taxtual aids: Braille writer slate and stylus.
• Auditory aids: Cassettes, tape recorders, talking books, etc.
• Electronic aids: Talking calculators and computers, CCTV (closed circuit television), etc.

Optical aids: Bifocal lenses, contact lenses, tinted lenses, magnifiers and telescopic aids.

Question 10.
What are the characteristics of image formed by a plane mirror?
Answer:
Characteristics of image formed by a plane mirror are as follows:

• Obtained image is always formed behind the mirror.
• Obtained image is virtual, i.e., it cannot be obtained on screen.
• Image is erect and laterally inverted.
• Image size is equal to object size.
• Distance of the object and image from the mirror is the same.

Long Answer Type Questions

Question 1.
How will you trace image formation in plane mirror?
Answer:
Image formation on plane mirror:

In this figure, PQ is the plane mirror and O is an object. OA and OC are the incident rays, coming from the object. The incident rays are reflected from the mirror and reflected rays emerge as AB and CD. If AB and CD are extended behind the mirror, they appear to meet at point ‘I’. This is the point where image is formed. If OM is object distance and MI is image distance, then in plane mirror, OM = MI. This means image distance is equal to object distance in case of a plane mirror.

Question 2.
Explain the structure of human eye.
Answer:
The human eye is roughly spherical in shape and has the following main parts:
a. Cornea: The eyeball is covered with a tough layer which is transparent on the front. This transparent portion is called cornea.

b. Iris: Iris is a thin circular structure. It works like the shutter of a camera. It controls the amount of light entering the eye. The colour of iris imparts distinct colour to the eyes of an individual. A person with blue iris has blue eyes.

c. Pupil: There is a hole in the centre of iris. This is called pupil. Light enters the eye through the pupil.

d. Lens: Lens is present behind the pupil. The lens in the human eye is a convex lens. The size and thickness of the lens change as per the distance of the object to be seen.

e. Retina: Retina is present at the back of the eye and marks the inner layer of the eyeball. Images are formed on the retina and thus it works like a screen. There are photosensitive cells in the retina. These cells are of two types:

• Cone cells: The cone cells are sensitive to bright light. They also give the sense of colour.
• Rod cells: The rod cells are sensitive to dim light.

f. Optic Nerve: The optic nerve emerges from the back of the eyeball. This nerve goes to the brain for inteipretation of the image.

g. Blind Spot: The junction of retina and the optic nerve is called the blind spot. There is no photosensitive cell at this spot, and hence no image is formed at this spot on the retina.

Question 3.
What is Braille system? Explain how the English alphabets are written in this
Answer:
Braille is a system that uses combinations of raised dots to spell letters and numbers. It is used by people who are blind or partially sighted to help them read and write. Braille is not actually a language. It is a system of writing. This system was invented by Louis Braille. Louis Braille was born in 1809 in France. He became blind due to an accident in childhood. He developed the Braille script in 1824.

This system has 63 raised dot patterns or characters. Each character or pattern represents a letter, a combination of letters, a common word or a grammatical sign. Dots are arranged in cells of two vertical rows of three dots each. A reader needs to touch the Braille script to read it.

Question 4.
What are the uses of a plane mirror?
Answer:
Uses of plane mirrors are as follows:
a. Plane mirrors are used as looking glasses at homes, by hair dressers, and many more.
b. Plane mirrors also have many scientific uses. In most of the measuring instruments, a plane mirror is placed under the scale and pointer. When the pointer moves, the image in the plane mirror also moves. Now, the eye is so placed that the pointer covers its own image, and thus the pointer and its image coincide. Thus the eye, the pointer and its image in the mirror are in the same line perpendicular to the scale. This helps in getting the correct reading.
c. Plane mirrors are also used in constructing periscope which is used in submarines.
d. Plane mirrors are used in solar cookers.
e. Plane mirrors are used to make kaleidoscope, a toy which produces beautiful patterns from coloured paper, pieces of glass or small coloured beads.
f. They are used in headlights of vehicles and in torches as reflectors.

Picture Based Questions

Question 1.
Draw the diagram of: (a) Regular reflection (b) Irregular reflection.
Answer:

Question 2.
Observe the figure and answer the following questions.

a. Identify and name the system shown here.
b. Who developed this system?
c. How many total characters are there in this system?
d. What kind of people are benefitted from this and how?
Answer:
a. Braille system.
b. Louis Braille.
c. 63
d. It helps the visually impaired people to read and write by feeling the raised dots and recognising the character.

These NCERT Solutions for Class 8 Science Chapter 15 Some Natural Phenomena Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Some Natural Phenomena NCERT Solutions for Class 8 Science Chapter 15

Class 8 Science Chapter 15 Some Natural Phenomena Textbook Exercise Questions and Answers

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Select the correct option in Questions 1 and 2.

Question 1.
Which of the following cannot be charged easily by friction?
a. A plastic scale
b. A copper rod
c. An inflated balloon
d. A woollen cloth
Answer:
b. A copper rod

Question 2.
When a glass rod is rubbed with a piece of silk cloth the rod
a. and the cloth both acquire positive charge
b. becomes positively charged while the cloth has a negative charge
c. and the cloth both acquire negative charge
d. becomes negatively charged while the cloth has a positive charge
Answer:
b. becomes positively charged while the cloth has a negative charge

Question 3.
Write T against true and F against false in the following statements:
a. Like charges attract each other. (T/F)
b. A charged glass rod attracts a charged plastic straw. (T/F)
c. Lightning conductor cannot protect a building from lightning. (T/F)
d. Earthquakes can be predicted in advance. (T/F)
Answer:
a. F,
b. T,
c. F,
d. F

Question 4.
Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain.
Answer:
When we take off a sweater, it rubs against our shirt. The rubbing together of sweater and shirt produces opposite electric charges on them. The discharge of these electric charges produces tiny sparks of light as well as crackling sound.

Question 5.
Explain why a charged body loses its charge if we touch it with our hand.
Answer:
Human body is a conductor of electricity. So, when we touch a charged object, our body conducts its charges to the earth. That is why a charged body loses its charge, if we touch it with our hand. This phenomenon is known as electric discharge.

Question 6.
Name the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is it likely to cause much damage?
Answer:
The destructive energy of an earthquake is measured by the Richter scale. The reading of magnitude 3 on the Richter scale would be recorded by a seismograph. If the Richter scale gives a reading of magnitude 3, the earthquake is not likely to cause much damage. Generally, earthquakes having magnitudes higher than 7 are destructive in nature.

Question 7.
Suggest three measures to protect ourselves from lightning.
Answer:
Protective measures against lightning are as follows:

• Stay in a completely closed place. If you are moving in a car, remain there till the lightning is over. Close the windows of the car immediately.
• Do not touch any electrical wires, telephone cables, metal pipes, etc.
• Do not take a bath in running water. This may cause an electric shock.

Question 8.
Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon?
Answer:
We know that like charges repel each other. Since both the balloons carry similar charges, they are repelled by each other. But a charged balloon attracts an uncharged balloon because a charged object has a tendency to attract an uncharged object.

Question 9.
Describe with the help of a diagram an instrument which can be used to detect a charged body.
Answer:
An electroscope can be used to detect whether a body is charged or not. The figure given here shows a simple electroscope.

It consists of a metal rod. At one end of the rod, two leaves of aluminium foil are fixed and at the other end, there is a metal disc. The leaves of aluminium foil are placed inside a closed jar to isolate the leaves from air.

When the metal disc is touched with a charged body, the aluminium strips move away from each other. This happens because some of the charges of the body are transferred to the strips through the metal rod. The nature of charges on both the leaves is similar. Hence, both the leaves of the aluminium foil move away from each other. If, however, the body is not charged, the two leaves remain as they are and will not repel each other.

Question 10.
List three states in India where earthquakes are more likely to strike.
Answer:
The three states in India where earthquakes are more likely to strike are Jammu and Kashmir, Gujarat and Assam.

Question 11.
Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself?
Answer:
If we are out in the open and an earthquake strikes, we can take the following precautions:

• We should try to move away from buildings and other structures.
• If we are in a vehicle, we should ask the driver to drive slowly to a safe spot.
• We should keep away from bridges.

Question 12.
The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out on that day, would you carry an umbrella? Explain.
Answer:
If a thunderstorm is being predicted, we should not carry an umbrella. The metallic handle of an umbrella can be a potential target for lightning, and therefore, it is unsafe to carry an umbrella.

NCERT Extended Learning Activities and Projects

Question 1.
Open a water tap. Adjust the flow so that it forms a thin stream. Charge a refill. Bring it near the water stream. Observe what happens. Write a short report on the activity.
Hint:
Even if the water molecules as a whole are neutral, one of their ends is negatively charged while the other end is positively charged. Once the water molecule comes in contact with a charged object, it will get attracted towards it.

Question 2.
Make your own charge detector. Take a paper strip roughly 10 cm × 3 cm. Give it a shape as shown in the figure. Balance it on a needle. Bring a charged body near it. Observe what happens. Write a brief report, explaining its working.

Hint:
When a charged body is brought near it, it starts moving. When the charged body is rotated in a circular way, it also shows a circular motion.

Question 3.
This activity should be performed at night. Go to a room where there is a fluorescent tube light. Charge a balloon. Switch off the tube light so that the room is completely dark. Bring the charged balloon near the tube light. You should see a faint glow.
Move the balloon along the length of the tube and observe how the glow changes.
Caution: Do not touch the metal parts of the tube or the wires connecting the tube with the mains.
Hint:
When a charged balloon touches the tubelight, electrons pass from the balloon to the tubelight causing the bulb to emit small sparks of light. Under normal circumstances, the tubelight receives electrons from the electric power lines through a wire at the end of the tube.

Question 4.
Find out if there is an organisation in your area which provides relief to those suffering from natural disaster. Enquire about the type of help they render to the victims of earthquakes. Prepare a brief report on the problems of the earthquake victims.
Hint:
Students should find out about the organisations in their respective areas which provide relief to those suffering from a natural disaster. An earthquake
may cause damage and destruction to life and property. Roads and bridges collapse. Earthquake victims have to face deficiencies of food, water, security, medicine, fuel and so on, apart from the loss of lives of their loved ones.

Activity 1

Objective: To collect the different objects and charge them by rubbing.
Materials Required: Refill, balloon, eraser, steel spoon, coin, ball pen, polythene, woollen cloth, dry hair.
Procedure and Observations:

• Collect the different articles listed above and rub them with the objects as shown in the table given below.
• Try to attract small bits of paper with them and observe what happens in each case.
• Use insulating gloves while charging the metal spoon and coin.

Conclusion: Objects can be charged by rubbing.

• Positive and Negative Charges: There are two types of charges namely positive charge and negative charge. Like charges repel each other while unlike charges attract each other.
• It is a convention to call the charge acquired by a glass rod when it is rubbed with silk as positive. The other kind of charge is said to be negative.

Activity 2

Objective: To show that like charges repel each other, whereas unlike charges attract each other.
Materials Required: Two glass rods, two ebonite rods, a piece of silk and a piece of fur.
Procedure:

• Rub a glass rod with a piece of siik and hang it with a thread (Fig. a).
• Rub another glass rod with a piece of silk and bring it near the suspended charged glass rod. See what happens.
• Rub an ebonite rod with a piece of fur and hang it with a thread (Fig. b).
• Rub another ebonite rod with a piece of fur and bring it near the suspended charged ebonite rod and observe.

Now perform similar activity by bringing a charged ebonite rod near a suspended charged glass rod (Fig. c).

Observation: Charged glass rod moves away s from another charged glass rod and charged ebonite rod moves away from another charged ebonite rod. But. a charged glass rod gets attracted towards a charged ebonite rod.
Conclusion: When the charges repel each other, they are like charges, and when the charges attract each other, they are unlike charges.

The electrical charges produced by rubbing are called static charges. When charges move, they constitute an electric current.

Transfer of Charge: Electric charge can be transferred from a charged object to another charged or uncharged object through a metal conductor.

Electroscope: It is a device that can be used to test whether an object is carrying any charge or not. An electroscope consists of two closely placed metallic (aluminium) foils or strips. When an electroscope is charged, its aluminium strips are diverged. If we touch the metal top of a charged electroscope with our finger, it gets discharged and its aluminum strips collapse. This is because the electric charge present on the charged electroscope flows to the earth through our hand and body. The electroscope loses all the electric charge, becomes uncharged and its diverged aluminium strips collapse.

Discharging: When a charged object loses its charge, it is said to be discharged. Earthing: When a charged object transfers its charge to the earth, it is called earthing. Generally, every building is provided with an earthing to protect it from electrical shocks due to leakage of electric current.

The Story of Lightning:
i. During the formation of a thunderstorm, the air currents move upward and the water droplets move downward. These vigorous movements result in separation of charges in the clouds.

ii. From these generated charges, the positive charges accumulate near the upper edges of the clouds and the negative charges accumulate near the lower edges of the clouds. Scientists are yet to understand the exact reason for this. There is an accumulation of positive charges near the ground also.

iii. When accumulation of charges becomes very large, the air which is a bad conductor of electricity under normal environment, is no longer able to resist their flow. As a result, the electric charges meet and produce streaks of bright light and sound across the sky. This process is called an electric discharge. It can occur between two or more clouds or between the clouds and the earth.

Dangers of Lightning: Lightning can damage houses and trees. It can also kill people and cattle.

Lightning Safety: One should not stay at an open place during lightning and thunderstorm. Hence as soon as we hear any alert about lightning or thunderstorm, we should rush to a safe place such as a house or a building. If somebody is there in a car or bus, they should stay inside and keep all the doors and windows closed. One should remain inside the safe place until the storm lasts.

Do’s and Don’ts during a Thunderstorm:
Outside the house-

• Open vehicles, like motorbikes, open cars, etc., are not safe.
• Open fields, tall trees, parks and elevated places are also not safe.
• Do not carry an umbrella during a thunderstorm.
• In a forest, take shelter under shorter trees.
• In an open field, stay away from trees, poles and other metal objects.
• Squat low on the ground, place your hands on your knees with your head between the hands. This will make you the smallest target to be hit.

Inside the house-

• Avoid contact with electrical switches and appliances.
• It is safer to use cell phones or cordless phones instead of a wired phone.
• Bathing should be avoided during thunderstorms to avoid contact with running water.
• Electrical appliances should be unplugged.

Lightning Conductor: This is a simple device which protects a building from the effect of lightning. It is composed of a vertical metallic rod which usually has a trident at the top and is taller than the building. The base of the metallic rod is attached to a thick metallic plate which goes very deep inside the earth. This metallic wire provides a passage for earthing. When lightning strikes, the electric charge goes to the lightning conductor which transfers it safely to the earth. Thus, a lightning conductor helps in preventing any damage to the building.

Earthquake: Sudden shaking of the earth which lasts for a very short duration is called earthquake. Earthquake can cause immense damage to buildings, dams, etc. They may also cause floods, landslides, tsunamis and loss of life.

A major earthquake occurred in India on 8 October 2005, in Uri and Tangdhar towns of North Kashmir. Before that, a major earthquake occurred on 26 January 2001, in Bhuj district of Gujarat.

Cause of an Earthquake: It is caused by a disturbance deep inside the earth’s crust. There are three layers of the earth-the outer crust, the middle mantle and the inner core. The outermost layer of the earth is not in one piece. It is fragmented and each fragment is called a plate or earth plate. These plates are in continuous motion. When these plates collide with one another, one plate goes under another plate which causes disturbance in the earth’s crust. Due to this disturbance, tremors take place in the form of earthquakes.

Seismic or Fault Zones: The earthquakes occur due to collision of plates. Therefore, the boundaries of plates are the weak zones where an earthquake is likely to occur. These week zones are called ‘Seismic zones’.

The places of seismic zones in India include Kashmir, Western and Central Himalayas, the whole North East, Great Rann of Kutch in the district of Gujarat, and some areas of South India.

Richter Scale: A scale which is used to determine the magnitude or strength of an earthquake is called the Richter scale. Destructive earthquakes have magnitude of more than 7 on a Richter scale.

Seismic Waves: The earthquakes produce tremors or waves on the surface of the earth, which are called seismic waves. These waves travel in all the directions on the earth’s surface.

Seismograph: A seismograph is an instrument which can record the seismic waves. It contains a metal rod or a pendulum which can vibrate as the earthquake occurs. The metal rod is attached to a pen which records the waves on the paper. Scientists study these waves and then construct a map of the earthquake. This also helps them in determining the power of the earthquake.

Protection against Earthquake: The buildings located in the seismic zones should be constructed in a way that they can handle major tremors. The following measures should be taken by the people to make the houses or buildings quake-safe:

• People should always consult architects and engineers before constructing a new building.
• The roofs of the building should be kept light.
• Timber or mud should be used in the construction of buildings rather than heavy materials.
• The shelves and cupboards should be fixed to the walls so that they do not fall off easily.
• Things such as photo frames, geysers, clocks and other wall hangings should be carefully placed so that they do not harm anybody if they fall off.
• Buildings should have firefighting equipment as earthquakes may cause fires.
• During the earthquake, one should take the following steps to protect oneself:

Inside the house:

• During earthquake, you should hide under a table. If you are in bed, keep a pillow over your head and do not move out of the bed.
• You should stay away from tall and heavy objects that may fall on you due to tremors.
• Stay away from tall and heavy objects that may fall on you.

Outside the house:

• In an open area, you should try to move away from buildings, trees, overhead power lines and other structures.
• If you are in a car or a bus, do not come out. Ask the driver to drive slowly to a clear spot. Do not come out till the tremors stop.

Class 8 Science Chapter 15 Some Natural Phenomena Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
Name some destructive natural phenomena.
Answer:
Earthquake, lightning, cyclones, etc.

Question 2.
What is earthing?
Answer:
Transfer of charge from a charged body to the earth is called earthing.

Question 3.
What is the main purpose of providing earthing in buildings?
Answer:
To protect the building from electrical shocks due to any leakage of electrical current.

Question 4.
What is an earthquake?
Answer:
Sudden shaking of the earth, which lasts for a very short duration, is called an earthquake.

Question 5.
Name the place where major earthquake occurred in 2005 in India.
Answer:
Uri and Tangdhar towns of North Kashmir.

Question 6.
What is tectonic plate?
Answer:
The crust of the earth is composed of several pieces of landmass. These pieces are called tectonic plates.

Question 7.
What are seismic zones?
Answer:
The boundaries of the tectonic plates are at a high risk of earthquakes. These boundaries are called seismic zones or fault zones.

Question 8.
What is the magnitude of highly destructive earthquakes on Richter scale?
Answer:
Higher than 7.

Question 9.
Why is the charging by rubbing known as static electricity?
Answer:
Charging by rubbing is known as static electricity because the charges acquired by rubbing do not move.

Question 10.
Give the difference between static electricity and electric current?
Answer:
In static electricity, the charges do not move whereas in electric current, there is a flow of charges.

Question 11.
What causes a tsunami?
Answer:
An earthquake or volcanic activity occurring under the ocean causes a tsunami.

Question 12.
Name the three distinct layers of earth.
Answer:
The outer crust, the middle mantle and the inner core.

Question 13.
What are seismic waves?
Answer:
The tremors produce waves on the surface of the earth. These are called seismic waves.

Question 14.
What is hypocenter or focus?
Answer:
It is the point deep within the earth from where seismic waves originate.

Question 15.
What is focal depth?
Answer:
Focal depth is the vertical distance between the hypocenter (focus) and the epicenter of an earthquake.

Question 16.
What is an electric discharge?
Answer:
The process of meeting of negative and positive charges to release a huge amount of energy is called electric discharge.

Question 17.
What are the harmful effects of lightning on a lightning victim?
Answer:
Loss of memory, loss of sight or hearing, broken bones, etc.

Question 18.
What is Richter scale?
Answer:
This is a logarithmic scale which shows the intensity of an earthquake.

Short Answer Type Questions

Question 1.
What do you understand by charging by rubbing? Give suitable examples.
Answer:
Some objects get electrically charged when they are rubbed against one another. For example, when a plastic refill is rubbed with a polythene, both of them become charged.

Question 2.
Explain the structure of lightning conductor.
Answer:
This is a simple device which protects a building from the effects of lightning. It is composed of a vertical metallic rod which usually has a trident at the top. The base of the metallic rod is attached to thick metallic plate, which is buried very deep inside the earth. This metallic wire provides a passage for electric discharge in the earth.

Question 3.
Explain the structure of seismograph.
Answer:
Seismograph is a device which records the seismic waves. It is composed of an oscillator, a writing device and a roll of paper. The writing device is attached to the oscillator. In case of an earthquake, the oscillator begins to oscillate. This creates oscillation in the writing device, which plots wave-like patterns on the paper. The wave-like pattern is then analysed by seismologists to interpret the various features of an earthquake.

Question 4.
Why do we say that the Richter scale is not a linear scale?
Answer:
An instrument used to measure the magnitude or intensity of an earthquake is called Richter scale. The number indicating the magnitude or intensity on Richter scale usually ranges from 0 to 9 and above. The Richter scale is not a linear scale because on this scale, the vibrations of an earthquake with a magnitude of 2 are 10 times greater in amplitude than those of an earthquake with a magnitude of 1.

Question 5.
List three effects of an earthquake.
Answer:
The effects of earthquakes include deformation of ground surface, damage and destruction of human-made structures, destruction of towns and cities, loss of life, violent devastating fires, landslides, floods, etc.

Question 6.
Why does lightning usually strike tall buildings?
Answer:
Static electric charge is induced in tall buildings during lightning. Taller the buildings are, closer they are to the clouds and the charges flowing down from the sky. Charge induction happens more in closer objects than in faraway objects. Hence, lightning usually strikes tall buildings as they are the easy and quick targets.

Question 7.
What were the things used in Benjamin Franklin’s kite experiment?
Answer:
He flew a kite made of silk and a metal key tied to it during a thunderstorm. He got electric shock on touching the metal key. This proved that there are charges in the cloud.

Question 8.
Why are thunderstorms usually caused during summer?
Answer:
Thunderstorms are usually caused by the intense heating of the ground surface during summer. The warm air rises rapidly to form such clouds which accumulate electrical charges. These charges when discharged to the ground as flashes of lightning, cause heating of the air which comes in their path. This heated air expands rapidly, creating shock waves that are heard as a crash or rumble of thunder.

Question 9.
It is safer to use a wireless telephone rather than using a landline telephone during lightning. Explain the statement with the help of a reason.
Answer:
Lightning is a process of electrical discharge. During lightning, atmospheric charges may get discharged through landline telephone wires and may become dangerous. Therefore, it is safer to use a wireless telephone instead of a landline telephone during lightning.

Question 10.
Suggest some tips for a quake-safe building.
Answer:
Following provisions make a quake-safe building:

• Buildings should be made by consulting engineers and architects.
• Light-weighted materials should be used for making the buildings in earthquake-prone areas.
• Cupboards should be fixed to the walls so that they do not fall on someone during an earthquake.
• Proper installation of fire-fighting equipment should be ensured in buildings. Fire due to short circuit and gas leakage is quite common during an earthquake.

Question 11.
Explain the safety measures to be taken during a thunderstorm when you are inside your home.
Answer:
Safety measures to be taken during a thunderstorm when you are inside the house are as follows:

• Lightning can strike telephone cords, electrical wires and metal pipes. During a thunderstorm, contact with these should be avoided.
• Bathing should be avoided during thunderstorm to avoid contact with running water.
• Electrical appliances like computers, TVs, etc., should be unplugged. Electrical lights can remain on. They do not cause any harm.

Long Answer Type Questions

Question 1.
Explain the formation of lightning.
Answer:
Formation of lightning takes place in the following steps:
a. During a thunderstorm, the air currents move upward and water droplets move downward. These movements take place at a very fast pace, and it results in the separation of charges in the clouds.
b. The positive charges accumulate at the upper edges of the clouds and the negative charges accumulate at the lower edges of the clouds. Scientists are yet to understand the exact reason for this.
c. At the same time, the ground becomes positively charged.
d. Air is a bad conductor of electricity under normal circumstances. But when the amount of charge becomes too much in the clouds, the air is no more able to hold on to this charge. As a result, the electric charge is transferred to the ground. This appears as a bright streak of light across the sky, which lasts for a few seconds.

Question 2.
Mention some safety tips during lightning.
Answer:
Safety tips during lightning are as follows:

• No open place is safe during lightning, and hence a home or any other building is the safest place during lightning.
• The first sound of thunder is like a warning call for lightning. Hence, one should rush to a building soon after hearing a sound of thunder. One should come out of home only a few minutes after the last sound of thunder.
• You should not carry an umbrella (with a metallic handle) during a thunderstorm. Metallic handle can be a potential target for lightning.
• Tall trees and other tall structures are more prone to be hit by lightning. If you are caught in the open, try to hide under a short tree.
• If you are in open and have no place to hide, crouch on the ground and keep your head between your knees and hands.
• Do not take shower during lightning.
• Use of phone should be avoided during lightning.
• Television should be disconnected from the antenna.
• Avoid operating the electric switches and unplug all electrical gadgets during lightning.

Question 3.
List the steps you should take during an earthquake.
Answer:
Following are some steps you should take if you are caught in an earthquake:

• If trapped in your home or a building, take shelter under a table and do not move till the shaking stops. Protect your head with your arms. Avoid using a lift.
• Do not stay near the windows, bookcases, mirrors, hanging pots, fans, etc.
• Leave your home or school building and move to the open areas.
• If outdoors, keep away from high-rise buildings, trees, signboards, electric poles and electric wires.
• If you are in a car or a bus, do not come out. Ask the driver to drive slowly to a clear spot. Do not come out till the tremors stop.

Question 4.
Explain the mechanism by which earthquakes are caused.
Answer:
The tremors are caused by the disturbance deep down inside the uppermost layer of the earth called the crust. The earth’s crust is made up of several pieces of landmass. These are called tectonic plates. These plates are in continuous motion. When they brush past one another or one plate goes under the other due to collision, they cause vibrations in the earth’s crust. These vibrations occur as an earthquake on the surface of the earth.

Picture-Based Questions

Question 1.
Observe the figure given below and answer the following questions.

a. Name the device shown.
b. What is its use?
c. Do the metal leaves inside the jar acquire like or unlike charges?
d. Is it based on attraction or repulsion?
Answer:
a. Electroscope.
b. It is used to detect the presence or absence of charge on a body.
c. Like charges.
d. Repulsion.

Question 2.
Observe the given figure and answer the questions.

a. Name the setup shown here.
b. What is its purpose?
c. Can it be made of an insulating rod?
Answer:
a. Lightning conductor.
b. It is used to protect the building from the effects of lightning.
c. No.

Question 3.
Draw a labelled diagram of a seismograph.
Answer: