Class 8 - Physics - Light

Question1.

Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.

Light enables us to see. In dark room, there is no light for reflection to take place; hence we can’t see.

We will be seeing the objects outside the room only if there is presence of light.

Question2.

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

In diffused reflection, the surface is rough. Laws of reflection holds true. Due to irregularities on surface, reflected rays scatter in different directions.

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

• A polished wooden table is a smooth surface. Regular reflection takes place.
• Chalk powder has irregular surface. Diffused reflection takes place.
• A cardboard has irregular surface. Diffused reflection takes place.
• A marble floor with water is a smooth surface. Regular reflection takes place.
• A mirror has smooth surface. Regular reflection takes place.
• A piece of paper may appear smooth but it has many minor irregularities. Diffused reflection takes place.

Question 4.

State the laws of reflection.

Laws of reflection:

1. The angle of incidence is always equal to the angle of reflection.
2. 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.

1. Fix a white sheet of stiff paper on a drawing board or a table.
2. Take a comb and close all its openings except one in the middle (Hold the comb perpendicular to the sheet of paper).
3. 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.
4. 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.
5. Cut the projecting portion of the sheet in the middle. Look at the reflected    Make sure that the reflected ray extends to the projected portion of   the paper.
6. Bend that part of the projected portion on which the reflected ray falls.
7. Bring the paper back to the original position.

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 ____________.

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

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

(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.

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

(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

(c) Under special conditions     (d) Never

Correct option is :- (a) Always, Angle of incidence is equal to the angle of reflection.

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

(c) real at the surface of the mirror and enlarged

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

Correct option is: - Image formed by a plane mirror is (b) virtual, behind the mirror and of the same size as the object.

Question 9.

Describe the construction of a kaleidoscope.

Construction of a kaleidoscope is as follows:-

1. Take 3 rectangular mirror strips about 15 cm long and 4 cm wide each.
2. Join them together to form a prism as shown in Fig (a).
3. Fix them in a circular cardboard tube or tube of a thick chart paper.(Make sure that the tube is slightly longer than the mirror strips)
4. Close one end of the tube by a cardboard disc having a hole in the centre, through which you can see [Fig(b)].
5. To make the disc durable, paste a piece of transparent plastic sheet under the cardboard disc.
6. At the other end, touching the mirrors, fix a circular plane glass plate [Fig. (c)].
7. Place on this glass plate several small pieces of coloured glass (broken pieces of coloured bangles).
8. Close this end of the tube by a ground glass plate. Allow enough space for the colour pieces to move around.

Question 10.

Draw a labelled sketch of the human eye.

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?

Teacher told not to do so as the intensity of laser beam is very high. Laser beam are very

harmful to our eyes which can cause permanent damage to our eyes. So we should not look through the laser beam directly or indirectly.

Question 12.

Explain how you can take care of your eyes.

Following are the points we should keep in mind in order to take good care of eyes:-

1. We should not look at the sun or a powerful light directly.
2. Wash our eyes frequently with clean water.
3. 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.
4. 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.

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?

Angle of incidence = Angle of reflection

angle i = angle r

But given angle i = angle r

angle i = angle i

2(angle i )= 900

(angle i )=(900 /2)

=450

Question 14.

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

Infinite images are formed when two plane mirrors are placed parallel to each other, irrespective of the distance.

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.

Draw the reflected ray from the second mirror.

First mirror:-

(angle i =300 )

Therefore (angle r = 300)

Second mirror:-

(angle i )=(900- 300) = 600

Therefore (angle r ) = 600

Question 16.

Boojho stands at A just on the side of a plane mirror as shown in Fig. Can he see himself in the mirror?

Also can he see the image of objects situated at P, Q and R?

Boojho cannot see his image because reflected ray won’t reach his eyes.

He can see the image of objects situated at P, Q because the rays coming from P and Q get reflected by mirror and reaches his eyes.

Boojho can’t see image of objects situated at R because the ray from object R does not get reflected.

Question 17.

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

(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’ = Position of image of A. The distance of A from the mirror is same as the distance of the A’ from the mirror.
• Yes Paheli at B can see the object because reflected ray from A will reach B.
• Boojho can also see the image because the reflected ray from A will reach him.
• Let Paheli is moving from position B to position C. Paheli will be able to see the image from both the points,
• because image position depends on A. Position of image A’ remains unchanged.