NCERT 7th Class (CBSE) Science: Light

Question: Why does dispersion of white light occur?

Answer: light of all colours travel at the same speed in vacuum. But in any transparent medium such as glass, water, atc., the lights of different colours travel with different speed. The red light travels of all the seven colours. As a result the red light bends the least and the violet light bend the most. This unequal bending of the lights of different colours causes dispersion of white light.

Question: What are the characteristics of images formed by a concave mirror?

Answer: The characteristics of images formed by a concave mirror are:

1. The image is not always the same size as the object. It can be larger or smaller depending on the distance of the object to the mirror.
2. The image is formed on the screen. Such type of image that can be formed on a screen is called real image.
3. The image is inverted when the object is kept far from the mirror. When it is brought very close to the concave mirror, an erect and enlarged  image is formed.

Question: What are the characteristics of images formed by a convex mirror?

Answer: The characteristics of images formed by a convex mirror are:

1. The image is erect.
2. The image is smaller than the object.
3. The image formed is virtual.

Question: Write two uses of convex mirror.

Answer: Convex mirror is used:

1. As a year view mirror in vehicles to help the driver check the view of the traffic behind them.
2. As a reflector in street lamps to spread light over a wider area.

Question: The mirror outside the driver seat of a vehicle is usually a convex mirror. Printed on such a mirror is usually the warning, “Vehicles in this mirror are closer than they appear.” What is the reason for this warning?

Answer: The reason for this warning is that the Virtual image produced in a convex mirror is reduced in size and therefore looks smaller, just as distant object will appear in a plane mirror. A driver, thinking that the side view mirror is a plane mirror, might incorrectly deduce from the small size of the image that the vehicle behind is far enough away to ignore.