With a neat labelled diagram, explain how a rainbow is formed.

Explanation

(1) The formation of a rainbow in the sky is a combined result of refraction, dispersion, internal reflection and again refraction of sunlight by water droplets present in the atmosphere after it has rained.

(2) The sunlight is a mixture of seven colours : violet, indigo, blue, green, yellow, organge and red. After it has stopped raining, the atmosphere contains a large number of water droplets. When sunlight in incident on a water droplet, there is (i) refraction and dispersion of light as it passes from air to water (ii) internal reflection of light inside the droplet and (iii) refraction of light as it passes from water to air.
(3) The refractive index of water is different for different colours, being maximum for violet and minimum for red. Hence, there is dispersion of light (separation into different colurs) as it passes from air to water.
(4) The combined action of different water droplets, acting like tiny prisms, is to produce a rainbow with red colour at the outer side and violet colour at the inner side. The remanining five colours lie between these two.
The rainbow is seen when the sun is behind the observer and water droplets in the front.