# Class 8 Science NCERT Solutions for Chapter – 16 Light

## Light

Question 1.
Suppose you are in the darkroom. Can you see objects in the room? Can you see objects outside the room? Explain.

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?

If the reflected rays are parallel to one another, the reflection is called regular reflection. The regular reflection takes place when incident rays fall on the smooth and plane surface like a plane mirror having no defect.

When the parallel incident rays fall on a plane mirror having rough or-irregular surfaces, after reflection the rays do not remain parallel. This reflection is called diffused reflection. No, diffused reflection is not the failure of the laws of reflection.

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.

Regular reflection will take place because the surface is polished.

(b) Chalk powder.

Diffused reflection will take place because it is a rough surface.

(c) Cardboard surface.

Diffused reflection will take place because it is not polished but a rough surface.

(d) Marble floor with water spread over it.

Diffused reflection will take place as marble and water present on the surface of marble together provide an irregular surface to light for reflection.

(e) Mirror.

Regular reflection takes place because a plane mirror is a polished surface.

(f) Piece of paper.

A piece of paper has a rough surface, so diffused reflection will take place.

Question 4.
State the laws of reflection.

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.

Questions 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.

Materials required: White sheet of stiff paper, a drawing board, drawing pins, a comb, a strip of a plane mirror.
Procedure :

1. A white sheet of stiff paper on a drawing board is fixed in such a way that sheet projects a little beyond the edge of the table.
2. A comb is taken and all its openings are closed except one in the middle.
3. A comb is held perpendicular to the sheet of paper.
4. Light is thrown from a torch through the opening of the comb from one side.
5. The torch and the comb are so adjusted that a ray of light along the paper on the other side of the comb is visible.
6. A strip of plane mirror is placed in the path of the light ray.
7. After striking the mirror, the incident ray is reflected, and the reflected ray is seen on the paper.
8. The projected portion of the .sheet is cut in the middle. It was made sure that the reflected ray extends to the projected portion of the paper.
9. The part of the projected portion on which the reflected ray fell was bent,
10. Try to see the reflected ray was made.
11. The paper was brought to its original position and again a ray was made to see the reflected ray again.

Observations: When the whole sheet of paper was spread on the table, it represented one plane. The incident ray, the reflected ray, and the normal at the point of incidence were seen on it. But when the sheet was bent, i.e., a plane different from the plane, in which the incident ray and the normal ray, was created. In this case, we were not able to see the reflected ray [See Fig. 16.4 (a) and (b) on Page 201, NCERT],

This activity shows that the incident ray, the normal at the point of incidence, and the reflected ray all 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.
2m

(b) 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
______.
Left Left hand

(c) The size of the pupil becomes ______ when you see it in dim light.
Large

(d) Night birds have ______ cones than rods in their eyes.
Less

Question 7.
The angle of incidence is equal to the angle of reflection.

(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never

(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.

(b) virtual, behind the mirror and of the same size as the object.

Question 9.
Describe the construction of a kaleidoscope.

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 a 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 labeled sketch of the human eye.

Question 11.
Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advice?

Laser torch can harm or even damage the eye. So, her teacher advised her not to do so.

Question 12.
Explain how you can take care of your eyes?

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. Too much light, like that of the sun, or powerful lamps, can injure the retina, which is very delicate.

(3) If advised, suitable spectacles should be used.

(4) We should wash our eyes frequently with clean water.

(5) We should always read from a normal distance for distinct vision and not by bringing the book too close or keeping it too far.

(6) 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.

(7) 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?

The angle of incidence is 90°.
(As the angle of incidence = Angle of reflection)

Question 14.
How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?

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 the figure. Draw the reflected ray from the second mirror.

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

Boojho will not be able to see himself in the mirror. But he can see images of objects situated at ‘P and Q and R.

Question 17.

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

(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?

(a) The image will be formed at the same distance behind the mirror.

(b) Yes

(c) Yes.

(d) The image remains at the same place. The image moves only if the object or the mirror is moved.

Question 1.

When light from an object enters our eyes, we see the object.

16.1 What makes Things Visible

Question 2.
Can you see an object in the dark?

No, for seeing an object light is also required.

Question 3.
Can you tell in which direction the light falling on a surface will be reflected?

The light ray, which strikes any surface, reflects in another direction.

16.2 Laws of Reflection Activity 16.1

Question 1.
What do you observe?

After striking the mirror, the ray of light is reflected in another direction.

Question 4.
Enter the data in Table 16.1 (Textbook page 200):

Question 5.
Do you see any relation between the angle of incidence and the angle of reflection? Are they approximately equal?

Yes, we can see a relation between the angle of incidence and the angle of reflection. They are approximately equal.

Question 6.
What would happen if I threw the light on the mirror along the normal?

The light will reflect along the normal only.

Activity 16.2

Question 1.
What do you infer?

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

Question 7.
Paheli asked him to recall those features :

(1) Was the image erector upside down?
It was erect.

(2) Was it of the same size as the object?
Yes.

(3) Did the image appear at the same? the distance behind the mirror as the object was in front of it?
Yes, the image formed was at the same distance behind the mirror as the object was in front of it.

(4) Could it be obtained on a screen?
No, it could not be. obtained on a screen, because it was a virtual image.

Activity 16.3

Question 1.
Extend them further. Do they meet? Extend them backward. Do they meet now?

No, the reflected rays do not meet on the further extension. On extending backward, the reflected rays meet at a point.

16.3 Regular and Diffused Reflection Activity 16.4

Question 1.
Are they parallel to one another?

No, the reflected rays are not parallel to one another.

Question 8.
I have a question. Can the reflected rays be further reflected in the incident on another mirror?

“Yes, reflected rays can be further reflected in the incident on another mirror.

16.4 Reflected Light can be Reflected Again

Question 9.
Do you know how you could see the hair at the back of your head?

The hack of the head can be seen due to the reflection of light from the mirror placed at the back of the head.

Question 10.
Can you explain how reflection from the two mirrors enables you to see objects which are not visible directly?

Yes, it is because reflected rays from one mirror are reflected again by another mirror. This phenomenon is also performed in a periscope.

16.5 Multiple Images

Question 11.
What happens if two plane mirrors are used in combination?

A number of images are formed if we place .two mirrors at an angle to one another.

Activity 16.5

Question 1.
How many images of the coin do you see (Fig. 16.10, Textbook page 204)?

Four

Question 12.
Can you now explain how you can see the back of your head at the hair dresser’s shop?

Yes. The back of the head can be seen due to the reflection of light from the mirror placed at the back of the head.

Activity 16.7

Question 1.
How can you explain this?

As the mirror and water together form a prism, it breaks up the light into its constituent seven colors on the paper.

Activity 16.8

Question 1.
Do you notice any change in the size of the pupil? In which case was the pupil larger? Why do you think it was so?

When the light from the torch falls on the pupil, its size becomes smaller to allow less amount of light to enter into the eye. When the light was not falling on the pupil, its size was bigger, to allow more light to enter into the eye for distinct vision. It happened so to control the amount of light entering into the eye.

Question 2.
In which case do you need to allow more light in the eye, when the light is dim or bright?

When the light is dim, we need to allow more light in the eye.

Question 3.
What kind of lens is thicker at the center?

Convex lens.

Activity 16.10

Question 1.
Do you see the bird in the cage?