Class 8 Science

Light

NCERT Solution

Part 1

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: It is not possible to see in a dark room because of absence of light. We know that we are able to see something when light falls on it. If there is light outside the room, then the objects outside the room can be seen.

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

Answer: Regular reflection happens from a highly smooth surface, while irregular reflection happens from a rough surface. Clear images are formed in case of regular reflection, while blurred images are formed in case of irregular reflection.

The laws of reflection are followed in every situation. Hence, diffused reflection does not mean 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.

Polished wooden table
Chalk powder
Cardboard surface
Marble floor with water spread over it
Mirror
Piece of paper

Answer: Regular reflection will only take place in case of a mirror. In other cases, irregular reflection will take place; because the surfaces are not smooth.

Question 4: State the laws of reflection.

Answer: The laws of reflection are as follows:

First Law of Reflection: The incident ray, the reflecting ray and normal at the point of incidence; all lie in the same plane.

Second Law of Reflection: Angle of incidence is always equal to the angle of reflection.

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: For this, take a chart paper, a torch and a plain mirror. Paste a black paper on the front of the torch and make a small hole at the centre of black cover. This will give you a thin beam of light.

• Keep the chart paper on a table in a way that some portion of the chart paper is outside the edge of the table. Keep the mirror vertically on the chart paper.
• Direct the beam of light on the mirror so that the incident ray is along the surface of the chart paper. Mark the incident ray, the normal and the reflected ray.
• After that, fold the portion of the chart paper which is jutting out of the table. Repeat the above step and observe the incident ray and reflected ray.
• It is observed that the reflected ray cannot be seen on the folded portion of the chart paper. The folded portion of the chart paper is in a different plane than the remaining part of the chart paper. This proves that the incident ray, the normal and the reflected ray; all lie in the same plane.

Question 6: Fill in the blanks:

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: (a) 2 m, (b) left, left, (c) more, (d) less number of

Question 7: Angle of incidence is equal to the angle of reflection

1. Always
2. Sometimes
3. Under special conditions
4. Never