Give an example of 'Natural Spectrum'. Explain the formation of a rainbow in sky with a neat labelled diagram.

Explanation

A rainbow is a natural spectrum appearing in the sky after a rain shower.
Rainbow is caused by dispersion of sunlight on refraction by tiny water droplets, present in the atmosphere.
It is always formed in the direction opposite to that of the Sun. So the rainbow is seen when the Sun is behind the observer.
Here, water droplets act like small prisms. They first refract and disperse the incident sunlight, then reflect it internally (not necessarily total internal reflection) and finally refract it again while it comes out of the raindrop.

This phenomenon is shown using schematic diagram in the figure, using only one water drop out of many water drops.
From the figure one can say that, the water drop causes internal reflection once and refraction twice of the incident ray of sunlight.
Due to dispersion of light and internal reflection, different colours reach the observer.s eye.
Conditions for observing a rainbow :
(a) after rainfall / at a water fountain.
(b) The Sun should be behind the observer.