CBSE Class 10 Science 2017 · Paper 31/1/1 · Question 24

(a) A ray $PE$ falls on face $AB$ of a glass prism and bends towards the normal; it travels through the prism along $EF$ and, on leaving face $AC$ at $F$, bends away from the normal and emerges along $FS$. Because the two faces are inclined, the emergent ray is turned towards the base. The angle between the incident ray produced forward and the emergent ray (produced backward) is called the angle of deviation, $\angle D$ (see the ray diagram, which also marks $\angle i$, $\angle r$, $\angle e$ and the angle of the prism $\angle A$). (b) White light is a mixture of seven colours. The refractive index of glass is different for different colours (light of each colour travels at a slightly different speed in glass), so each colour bends through a different angle with respect to the incident ray as it passes through the prism: red bends the least and violet the most. The colours therefore emerge along different paths and become distinct, forming the spectrum VIBGYOR. This splitting is called dispersion. (c) A rainbow is formed by tiny water droplets in the air after a shower, each acting like a small prism. Sunlight entering a droplet is refracted and dispersed, is reflected internally at the back of the droplet, and is refracted again as it comes out. The colours emerge at slightly different angles, so an observer with the Sun behind sees red on the outer edge and violet on the inner edge of a bow in the sky opposite the Sun (see the labelled ray diagram).