NCERT Class VIII Science Solutions: Chapter 16-Light Part 2

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Question 4:

State the laws of reflection.

Answer:

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:

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.

Result for Laws of reflection:

Laws of Reflection

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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:

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

  2. 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 ____________.

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

  2. 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.

  3. The size of the pupil becomes large when you see in dim light.

  4. 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

  1. Always

  2. Sometimes

  3. Under special conditions

  4. Never

Answer

A. Always

Question 8:

Choose the correct option: Image formed by a plane mirror is

  1. virtual, behind the mirror and enlarged

  2. virtual, behind the mirror and of the same size as the object

  3. real at the surface of the mirror and enlarged

  4. 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