CBSE Class 8

NCERT Solutions for Class 8 Science Chapter 16 Light are part of NCERT Solutions for Class 8 Science. Here we have given NCERT Solutions for Class 8 Science Chapter 16 Light.

NCERT Solutions for Class 8 Science Chapter 16 Light

NCERT TEXTBOOK EXERCISES

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.
We cannot see objects in the darkroom. But, we can see objects outside the room, in case there is light outside the room. It is so because to see an object, we need light which gets reflected from the object and enters our eyes.

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

No, diffused reflection does not mean, failure of the laws of reflection. It is due to irregularities in the reflecting surface, like that of cardboard.

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.

1. Polished wooden table
2. Chalk powder
3. Cardboard surface
4. Marble floor with water spread over it
5. Mirror
6. Piece of paper

Answer.

1. Polished Wooden Table: Regular reflection will take place. This is because a polished wooden table will have a plane surface.
2. Chalk Powder: Diffused reflection because the surface of the chalk powder is not smooth.
3. Cardboard Surface: Diffused reflection since cardboard has a rough surface,
4. Marble Floor with Water Spread Over it: Regular reflection as it will act as a plane surface.
5. Mirror: Regular reflection because the mirror has a shiny surface which is even.
6. Piece of Paper: Diffused reflection because the surface of the paper is rough.

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

1. The angle of reflection is equal to the angle of incidence.
2. The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane.

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.

Question 6.
Fill in the blanks in the following:

1. A person 1 m in front of a plane mirror seems to be…….. m away from his image.
2. If you touch your………..ear with your right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with
3. The size of the pupil becomes………………..when you see in dim light.
4. Night birds have…………..cones than rods in their eyes.

Answer.

1. 2
2. left, left hand
3. large
4. lesser (fewer).

Choose the correct option in Questions 7 and 8.

Question 7.
The 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.
The 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 an optical instrument used to see a number of beautiful patterns. It is made up of a circular cardboard tube or tube of thick chart paper in which rectangular mirror strips are joined together to form a prism. At one end of the tube, touching these mirrors, a circular plane glass plate is fixed and several, small pieces of colored glass (broken pieces of colored bangles) are placed upon it. This end is closed by a ground glass plate and beautiful patterns are seen through the other end of the kaleidoscope.

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

Question 11.
Gurmit wanted to perform, activity 16.8 (NCERT) using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advice?
Answer.
Laser rays can cause a permanent defect in the eye. Thus, his friend will lose her eyesight if a laser torch is directed over her eyes.

Question 12.
Explain how you can take care of your eyes.
Answer.
The following measures should be taken to keep our eyes healthy and free from any defect:

1. We should not look at the sun or a powerful light source directly.
2. We should not try to see in too little or too much light. It is bad for the eyes. Insufficient light causes eye strain and headaches.
3. Too much light, like that of the sun, or powerful lamps, can injure the retina, which is very delicate.
4. If advised, suitable spectacles should be used.
5. We should wash our eyes frequently with clean water.
6. We should always read from a normal distance for distinct vision and not by bringing the book too close or keeping it too far.
7. We should never rub our eyes if any small particle of dust goes into the eyes. Instead, eyes should be washed with clean water and if the condition does not improve, a doctor should be consulted.
8. If food is deficient in some components, the eye may also suffer. Lack of vitamin A in our diet is responsible for many eye troubles. Most common amongst thorns is night blindness. Therefore, to get rid of this, we must include in our diet components that have vitamin A.

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.

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.
The infinite number of images of the candle will be formed if it is placed between two parallel plane mirrors.

Question 15.
Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 16.5. Draw the reflected ray from the second mirror.

Answer.

Question 16.
Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.6.
Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?

Answer.
No, He can see images of objects at P but not of Q and R.

Question 17.
(a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.7).
(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) Fig. 16.7 (a)
(b) Yes
(c) Yes
(d) No change as the position of A is not changing

We hope the NCERT Solutions for Class 8 Science Chapter 16 Light helps you. If you have any query regarding NCERT Solutions for Class 8 Science Chapter 16 Light, drop a comment below and we will get back to you at the earliest.

NCERT Solutions for Class 8 Science Chapter 15 Some Natural Phenomena are part of NCERT Solutions for Class 8 Science. Here we have given NCERT Solutions for Class 8 Science Chapter 15 Some Natural Phenomena.

NCERT Solutions for Class 8 Science Chapter 15 Some Natural Phenomena

NCERT TEXTBOOK EXERCISES

Select the correct option in Questions 1 and 2.

Question 1.
Which of the following cannot be changed easily by friction?
(a) A plastic scale
(b) copper rod
(c) An inflated balloon
(d) A woolen cloth.
Answer.
(b) A copper rod.

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

Question 4.
Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain.
Answer.
The crackling sound is heard because when the sweater is rubbed while taking it off, it acquires a small charge.

Question 5.
Explain why a charged body loses its charge if we touch it with our hand.
Answer.
When we touch a charged body the total charge is transferred into the earth through our body and it loses its charge.

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 on a scale called the Richter scale. An earthquake that measures 3 on the Richter scale Will be recorded by a seismograph. However, it is not likely to cause much damage.

Question 7.
Suggest three measures to protect ourselves from lightning.
Answer.

1. We should not use TV or cable phone during lightning.
2. After hearing the last thunder, wait for at least 30 minutes before coming out of the safe place.
3. A house or a building is a safe place. If you are travelling by car or by bus, you are safe inside with the windows and doors of the vehicle shut.

Question 8.
Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon?
Answer.
A charged balloon is repelled by another charged balloon because both have the same charge. And we know similar charges repel each other. But a charged balloon attracts an uncharged balloon and loses its own charge to the other balloon.

Question 9.
Describe with the help of a diagram an instrument which can be used to detect a charged body.
Answer.
Electroscope: An electroscope is a device used to detect the presence of charge of an object.
Principle: It works on the principle that like charges repel while unlike charges attract each other.

Structure and Working: An electroscope has a metal rod with a thin metal strip or leaf attached to it at the bottom. At the top, the rod enters into a cup. The bottom part of the rod and leaf is enclosed in a glass box for protection. When the knob of the electroscope is touched with a charged ebonite or glass rod, the leaves open out or diverge. The extent of divergence depends upon the amount of charge on the electroscope.

Question 10.
List three states in India where earthquakes are more likely to strike.
Answer.
The states that fall in seismic zones are more earthquake-prone. Jammu and Kashmir, Himachal Pradesh, and Gujarat are a few such states.

Question 11.
Suppose you are outside your home and an earthquake strikes. What precautions would you take to protect yourself?
Answer.

1. Find a safe place, that will be away from buildings, trees and overhead power lines. Do not be on the ground but squat low on it to make yourself the smallest target to be struck.
2. If we are in a car or a bus, we should not come out, as the driver needs to drive slowly to a clear spot. Do not come out till the earthquake stops.

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.
No, I would not carry an umbrella. Because the pointed rod of the umbrella can become a target of lightning.

We hope the NCERT Solutions for Class 8 Science Chapter 15 Some Natural Phenomena help you. If you have any query regarding NCERT Solutions for Class 8 Science Chapter 15 Some Natural Phenomena, drop a comment below and we will get back to you at the earliest.

NCERT Solutions for Class 8 Science Chapter 14 Chemical Effects of Electric Current are part of NCERT Solutions for Class 8 Science. Here we have given NCERT Solutions for Class 8 Science Chapter 14 Chemical Effects of Electric Current.

NCERT Solutions for Class 8 Science Chapter 14 Chemical Effects of Electric Current

NCERT TEXTBOOK EXERCISES

Question 1.
Fill in the blanks:

1. Most liquids that conduct electricity are solutions of……..,……………and …………..
2. The passage of an electric current through a solution causes…………..effects.
3. If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the………..terminal of the battery.
4. The process of depositing a layer of any desired metal on another material by means of electricity is called.………

Answer.

1. acids, bases, salts
2. chemical
3. negative
4. electroplating.

Question 2.
When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?
Answer.
Yes, it is because the solution conducts electricity.

Question 3.
Name three liquids, which when tested in the manner shown in Fig. 14.1, may cause the magnetic needle to deflect.

Answer.
Tap water/sodium chloride solution, hydrochloric acid.

Question 4.
The bulb does not glow in the setup shown in Fig. 14.2. List the possible reasons. Explain your answer.
Answer.
List of possible reasons:

1. The liquid solution might be conducting electricity, but the current produced is too small so that the filament of the bulb does not get heated and the bulb does not glow.
2. Or, it is possible that the connections are loose.
3. Or, the bulb is fused.
4. Or, the cells are used up.
   

Question 5.
A tester is used to check the conduction of electricity through two liquids, labelled A and B. It is found that the bulb of the tester glows brightly for liquid A while it glows very dimly for liquid B. You would conclude that
(i) liquid A is a better conductor than liquid B.
(ii) liquid B is a better conductor than liquid A.
(iii) both liquids are equally conducting.
(iv) conducting properties of liquid cannot be compared in this manner.
Answer.
If the bulb glows brightly with liquid A, it means that liquid A is a better conductor of electricity than B. So the answer will be (i).

Question 6.
Does pure water conduct electricity? If not, what can we do to make it conducting?
Answer.
Pure water does not conduct electricity. It can be made conducting if acid, base, or salt is dissolved in it.

Question 7.
In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply of the area. Explain why they do this.
Answer.
The water used in water hoses is not pure water and it conducts electricity. Firemen shut off the main electrical supply of the area because if the supply of electricity continues there may be a high risk of electrocution in the whole area due to water.

Question 8.
A child staying in a coastal region test the drinking water and also the seawater with his tester. He finds that the compass needle deflects more in the case of seawater. Can you explain the reason?
Answer.
The impure water is the conductor of electricity. Seawater contains more impurity (salt) than drinking water. Therefore, seawater conducts more electricity than drinking water. That is why the compass needle deflects more in the case of seawater.

Question 9.
Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpours? Explain.
Answer.
It is not safe for the wireman to carry out electrical repairs outside during heavy downpours. Because during heavy downpour there is a high risk of electrocution.

Question 10.
Paheli had heard that rainwater is as good as distilled water. So she collected some rainwater in a clean glass tumbler and tested it using a tester. To her surprise, she found that the compass needle showed deflection. What could be the reasons?
Answer.
Rainwater is, of course, as good as distilled water but, when it passes through the atmosphere, it dissolves a lot of dust, dirt, and impurities and becomes conductive. So, when Paheli used a tester, its compass showed deflection.

Question 11.
Prepare a list of objects around you that are electroplated.
Answer.
Ornaments, wheel rims of vehicles, the handlebar of cycle and motorcycle, pots of one metal coated with other, bath taps, kitchen gas burner, the bottom of cooking utensils, handles of doors, tin cans, etc.

Question 12.
The process that you saw in Activity 14.7 (NCERT) is used for the purification of copper. A thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from impure rod is sought to be transferred to the thin copper plate. Which electrode should be attached to the positive terminal of the battery and why?
Answer.
To perform such activity we have to arrange copper sulphate crystals and two copper plates each having a size around 10 cm long and 4 cm wide. Now take 250 mL distilled water in a clean and dry beaker. Dissolve two teaspoonfuls of copper sulphate crystals in it to obtain copper sulphate solution. (You may add few drops of dilute sulphuric acid to copper sulphate solution to make it more conducting). Clean each copper plate with sandpaper. Now rinse it with water. With the help of wires, connect one copper plate to each terminal of two cell batteries (Take care that the two plates do not touch each other).

Immerse two plates in copper sulphate solution as shown in Fig. 14.3. Allow the current to pass through for 5 to 7 minutes.

When an electric current is passed through the copper sulphate solution, copper sulphate dissociates into copper and sulphate. The free copper gets drawn to the plate connected to the negative terminal of the battery and thus gets deposited on that plate. Gradually, a layer of copper builds upon the plate.

We hope the NCERT Solutions for Class 8 Science Chapter 14 Chemical Effects of Electric Current help you. If you have any query regarding NCERT Solutions for Class 8 Science Chapter 14 Chemical Effects of Electric Current, drop a comment below and we will get back to you at the earliest.

NCERT Solutions for Class 8 Science Chapter 13 Sound are part of NCERT Solutions for Class 8 Science. Here we have given NCERT Solutions for Class 8 Science Chapter 13 Sound.

NCERT Solutions for Class 8 Science Chapter 13 Sound

NCERT TEXTBOOK EXERCISES

Question 1.
Choose the correct answer:
Sound can travel through
(a) gases only
(b) solids only
(c) liquids only
(d) solids, liquids and gases
Answer.
(d) solids, liquids and gases.

Question 2.
Voice of which of the following is likely to have minimum frequency?
(a) Baby girl
(b) Baby boy
(c) A man
(d) A woman
Answer.
(b) Baby boy.

Question 3.
In the following statements, tick ‘T’ against those which are true, and ‘F’ against those which are false:

1. Sound cannot travel in a vacuum. (T/F)
2. The number of oscillations per second of a vibrating object is called its time period. (T/F)
3. If the amplitude of vibration is large, the sound is feeble. (T/F)
4. For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)
5. The lower the frequency of vibration, the higher is the pitch. (T/F)
6. Unwanted or unpleasant sound is termed music. (T/F)
7. Noise pollution may cause partial hearing impairment. (T/F)

Answer.

1. True
2. False
3. False
4. True
5. False
6. False
7. True

Question 4.
Fill in the blanks with suitable words:

1. Time taken by an object to complete one oscillation is called…….
2. Loudness is determined by the………..of vibration.
3. The unit of frequency is………….
4. Unwanted sound is called……….
5. The shrillness of a sound is determined by the.……….of vibration.

Answer.

1. time period
2. amplitude
3. Hertz (Hz)
4. noise
5. frequency.

Question 5.
A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer.

Question 6.
The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?
Answer.
Number of vibrations per second = 500
Time period = time is taken for one vibration

Question 7.
Identify the part which vibrates to produce sound in the following instruments:

1. Dholak
2. Sitar
3. Flute

Answer.

1. Stretched membrane
2. String of sitar
3. Air column.

Question 8.
What is the difference between noise and music? Can music become noise sometimes?
Answer.
The unpleasant sound is called noise, whereas a pleasant sound is called music. Noise can produce so many health hazards, whereas music brings about a soothing effect. Yes, music can become noise sometimes when the musical instruments produce very high volume sounds.

Question 9.
List sources of noise pollution in your surroundings.
Answer.
Following are the major sources of noise pollution:

1. Sounds of vehicles.
2. Sounds of the bursting of crackers and explosives.
3. Sounds of loudspeakers, TVs, transistors.
4. Sounds of kitchen appliances.
5. Sounds of desert coolers, air conditioners etc.

Question 10.
Explain in what way noise pollution is harmful to humans.
Answer.
Noise pollution may cause many health-related problems.

• Lack of sleep, hypertension (high blood pressure), anxiety, etc. are some of the problems that may be caused due to noise pollution.
• A person who is exposed to a loud sound continuously may get temporary or even permanent deafness.

Question 11.
Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.
Answer.
I would suggest my parents buy the house which is three lanes away from the roadside. This would protect us from noise pollution which is maximum on the roadside.

Question 12.
Sketch the larynx and explain its function in your own words.
Answer.
In humans, the sound is produced by the voice box or the larynx. It is situated in the throat. Below it is the windpipe. The two vocal cords are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air when the lungs force air through the slit, the vocal cords vibrate and the vibration results in sound.

Question 13.
Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why?
Answer.
The light travels at the speed of 3 x 10s ms-1, which is very large in comparison to the speed of sound which travels at the rate of 330 ms-1 in the air. That is why lightning is seen earlier and thunder is heard later.

We hope the NCERT Solutions for Class 8 Science Chapter 13 Sound helps you. If you have any query regarding NCERT Solutions for Class 8 Science 13 Sound, drop a comment below and we will get back to you at the earliest.

NCERT Solutions for Class 8 Science Chapter 12 Friction are part of NCERT Solutions for Class 8 Science. Here we have given NCERT Solutions for Class 8 Science Chapter 12 Friction.

NCERT Solutions for Class 8 Science Chapter 12 Friction

NCERT TEXTBOOK EXERCISES

Question 1.
Fill in the blanks:

1. Friction opposes the…………….between the surfaces in contact with each other.
2. Friction depends on the.…………..of surfaces.
3. Friction produces ……………….
4. A sprinkling of powder on the carrom board.…………….friction.
5. Sliding friction is……….than the static friction.

Answer.

1. relative motion
2. smoothness (or irregularities or nature)
3. heat
4. reduces
5. less.

Question 2.
Four children were asked to arrange forces due to rolling, static, and sliding frictions in decreasing order. Their arrangements are given below. Choose the correct arrangement.
(a) rolling, static, sliding
(b) rolling, sliding, static
(c) static, sliding, rolling
(d) sliding, static, rolling
Answer.
(c) static, sliding, rolling.

Question 3.
Alida runs her toy car on the dry marble floor, wet marble floor, newspaper, and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be
(a) wet marble floor, dry marble floor, newspaper, and towel,
(b) newspaper, towel, dry marble floor, wet marble floor,
(c) towel, newspaper, dry marble floor, wet marble floor,
(d) wet marble floor, dry marble floor, towel, newspaper.
Answer.
(a) wet marble floor, dry marble floor, newspaper, and towel.

Question 4.
Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.
Answer.
The frictional force will act parallel to the inclined surface, opposite to the direction of the sliding of the book.

Question 5.
You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?
Answer.
It is difficult to walk on a soapy floor because a soapy floor applies very little friction and hence we can slip on such floors.

Question 6.
Explain why sportsmen use shoes with spikes.
Answer.
Sportsmen use shoes with spikes because spikes produce the desired frictional force and thus help in holding the ground firmly.

Question 7.
Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?
Answer.
We know that the force of friction is increased if the two surfaces are pressed harder. A heavier box will apply more pressure on the floor and hence Seema will experience more frictional force.

Question 8.
Explain why sliding friction is less than static friction.
Answer.
Friction is caused by the interlocking of irregularities in the two surfaces. When the object starts sliding, the contact points on its surface, do not get enough time to lock into the contact points of the other surface. So, the sliding friction is slightly smaller than the static friction.

Question 9.
Give examples to show that friction is both a friend and a foe.
Answer.

Friction is both friend and foe, that is why it is said that friction is a necessary evil.

1. Friction as a friend:

• To hold a glass, we have ridges on our palm, which increase the friction between palm and glass.
• We are able to write anything with a pen or pencil because there is friction between the surfaces of paper and the point of a pen (nib) or pencil.
• A teacher can write on the blackboard with chalk because of the friction between the black-board surface and the chalk.
• If there is no friction, then a moving body would never stop.
• A nail could not be fixed in the wall, or a knot could not Be tied, had there been any friction.
• No building could be constructed without friction.

2. Friction as a foe:

• Friction wears out materials, whether they are screws, ball-bearings, or soles of shoes.
• Friction can also produce heat, which increases the wear and tear of machine parts. It also causes much wastage of energy, because this heat is not utilized.

Question 10.
Explain why objects moving in fluids must have special shapes.
Answer.
Birds and the fishes have to move about in fluids all the time. Thus, their “bodies have evolved to shapes which make them not to lose much energy in overcoming fluid friction. Such shapes are called streamlined. Cars are also designed so that their shapes resemble the streamlined shapes of birds, fishes, etc.

We hope the NCERT Solutions for Class 8 Science Chapter 12 Friction help you. If you have any query regarding NCERT Solutions for Class 8 Science Chapter 12 Friction, drop a comment below and we will get back to you at the earliest.