Chapter 16: Light

Science

(www.olympiadsuccess.com)

Chapter 16: Light

Class: VIII

NCERT Solutions

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 1

We can see any object, when light reflected by that object reaches to our eyes. But in the dark room, no light is reflected by the object. Hence, we are not able to see the objects in dark room. But if there is light outside the room, we can see the objects lying out there.

Question 2

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

Answer 2

S.No	Regular Reflection	Diffused Reflection
1.	It takes place from a smooth place	It takes place from a rough surface.
2.	Reflected rays moves in a particular direction.	Reflected rays scattered in random directions.
3.	Example: Reflection by plane mirror	Reflection by road surface.
4.

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

(e) Mirror (f) Piece of paper

Answer 3

(a) A polished wooden table is an example of smooth surface. So, when a beam of light reflects from it, a regular reflection will take place.

(b) Chalk powder has irregular surface. So, when a beam of light reflects from it, a diffused reflection will take place.

(d) A marble floor with water spread over it provides a smooth surface. So, when a beam of light reflects from it, a regular reflection will take place.

(e) A mirror has a smooth surface. So, when a beam of light reflects from it, a regular reflection will take place.

(f) A piece of paper may appear smooth but it has many minor irregularities. So, when a beam of light reflects from it, a diffused reflection will take place.

Question 4

State the laws of reflection.

Answer 4

Laws of reflection:

The angle of incidence is always equal to the angle of reflection.
The incident ray, the reflected ray and the normal to the surface at the point of incidence 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 5

Fix a white sheet of stiff paper on a drawing board or a table. Take a comb and close all its openings except one in the middle. Hold the comb perpendicular to the sheet of paper. Throw light from a torch through the opening of the comb from one side. With slight adjustment of the torch and the comb we will see a ray of light along the paper on the other side of the comb. Keep the comb and the torch steady. Place a strip of plane mirror in the path of the light ray. Let the sheet project a little beyond the edge of the Table. Cut the projecting portion of the sheet in the middle. Look at the reflected ray. Make sure that the reflected ray extends to the projected portion of the paper. Bend that part of the projected portion on which the reflected ray falls. Bring the paper back to the original position.

ncert

When the whole sheet of paper is spread on the table, it represents one plane. The incident ray, the normal at the point of incidence and the reflected ray are all in this plane. When we bend the paper you create a plane different from the plane in which the incident ray and the normal lie. Then we do not see the reflected ray. It indicates 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 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 ____________.

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

Answer 6

(a) A person 1 m in front of a plane mirror seems to be 2 m from his image.

(b) If you touch your left ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with left hand.

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

Question 7

Choose the correct option: Angle of incidence is equal to the angle of reflection

(a) Always (b) Sometimes

Answer 7 (a) Always

Question 8

Choose the correct option: 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

(d) real, behind the mirror and of the same size as the object.

Answer 8

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

Question 9

Describe the construction of a kaleidoscope.

Answer 9

A kaleidoscope is made up of three rectangular mirror strips joined together to form a prism. These are fix in a circular cardboard like tube or tube of a thick chart paper. The tube is slightly longer than the mirror strips. One end of the tube remains closed by a cardboard disc having a hole in the centre, through which one can see. To make the disc durable, a piece of transparent plastic sheet is pasted under the cardboard disc. At the other end, touching the mirrors, a circular plane glass plate is fixed. On this glass plate several small pieces of coloured glass is placed to form multiple images.

ncert

Question 10

Draw a labelled sketch of the human eye.

Answer 10

ncert

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?

Answer 11

Intensity of laser beam is very high, as it carries large amount of energy. It is harmful for eyes and can cause permanent damage. One should not look at laser beam directly or indirectly for a longer period.

Question 12

Explain how you can take care of your eyes.

Answer 12

It is necessary that we take proper care of our eyes. If there is any problem we should go to an eye specialist. Have a regular check-up.

If advised, we should use suitable spectacles
Too little or too much light is bad for eyes. Insufficient light causes eyestrain and headaches. Too much light, like that of the sun, a powerful lamp or a laser torch can injure the retina
We should not look at the sun or a powerful light directly.
Never rub our eyes. If particles of dust go into our eyes, wash our eyes with clean water. If there is no improvement go to a doctor.
Wash our eyes frequently with clean water.
Always read at the normal distance for vision. Do not read by bringing your book too close to our eyes or keeping it too far.

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 13

We know that angle of incident ray is equal to angle of reflected ray

. ⇒ ∠