NCERT 7th Class (CBSE) Science: Light

Question: Draw ray diagrams to represent the image formed by a convex lens and a concave lens when the object is kept between F and 2F.

Answer: 

Question: Show diagrammatically how a convex lens produces a magnified and virtual image.

Answer: 

Question: Distinguish between a concave and convex mirror.

Answer: Concave mirror:

1. It reflects light from its inner shining surface.
2. It can form both real and virtual image.
3. It can form both erect and inverted image.
4. It can form a magnified image.

Convex mirror:

1. It reflects light from its outer shining surface.
2. It always forms a virtual image.
3. It always forms an erect image.
4. It always forms a diminished image.

Question: What happens when a sharp image of the sun is produced on a paper?

Answer: By using a concave mirror, then after a few minutes, the paper starts at the point of sun’s image and a hole is formed in it. This is so because the concave mirror converges a lot of sun’s heat rays to a small point on paper.

Question: State the uses of concave mirrors.

Answer: Concave mirrors are used:

1. As reflectors in torches, vehicle head lights and search lights to get powerful beams of light.
2. As shaving mirrors to see a large image of the face.
3. By dentists to see the large image of the teeth of the patients.
4. In the field of solar energy to focus sun’s rays for heating solar furnaces.

Question: For driving a car what type of mirror would you prefer to see the traffic at your back?

Answer: We prefer convex mirror for observing the traffic behind us because its field of view is much larger than the plane mirror. However, it gives erroneous idea about the speed of the vehicles behind us. But because of the first advantage, we still prefer it.

Question: In a trick mirror, the image of head appear much smaller than that of the mouth and of nose. Explain why?

Answer: A trick mirror is basically a concave mirror having small focal length. The head is comparatively away from the nose and mouth. So, the magnification of head is smaller than that of nose and mouth. This is the reason that nose and mouth appear to be large in size than the head.