NCERT 7th Class (CBSE) Science: Light

Question: What is a spherical mirror? Distinguish between a concave mirror and a convex mirror.

Answer: A spherical mirror is that mirror whose reflecting surface is the part of a hollow sphere of glass.

1. Concave mirror: A concave mirror is the spherical mirror in which the reflection of light takes place at the concave surface or bent in surface. The reflection of light takes place from the concave surface. The back side of concave mirror has a silver coating which is protected by a layer of red paint. The back side of concave mirror is represented by drawing short, oblique lines.
2. Convex mirror: A convex mirror is that spherical mirror in which the reflection of light takes place at the convex surface (bulging out surface).

The convex surface of this mirror is shown in diagram. The reflection of light takes place from the convex surface. The back side of a convex mirror has a silver coating which is protected by layer of red paint. The back side of convex mirror is represented by drawing short, oblique lines.

Question: What is a rainbow? How is rainbow produced? What information do you get about sun light from the formation of a rainbow?

Answer: Rainbow: The rainbow is an arch of seven colours seen in the sky.

Just after rain, there are large number of small water drops suspended in the atmosphere. Each of these water drops acts like a small prism. As the white sunlight enters and leaves these tiny water drops, the various coloured rays of sunlight are deviated by different amounts and a long arch of seven colours formed in the sky called rainbow.

The formation of rainbow having seven colours shows that white light consists of seven colours.

Question: What is dispersion of light? Why does it take place?

Answer: The phenomenon of splitting of white light into its seven component colours is called dispersion of light. Splitting of white light into its component colours is due to the fact that lights of different colours travel with different speeds.

Question: Draw a diagram to show the dispersion of white light into seven colours by a glass prism.

Answer: Diagram is as follows:

Question: What is meant by spectrum? Name the colours of spectrum in their order.

Answer: The band of seven colours dispersed by a glass prism is known as a spectrum. The seven colours in a spectrum are violet, indigo, blue, green, yellow, orange and red known VIBGYOR.