What is a spectrum ? Why do we get a spectrum of seven when while light is dispersed by a prism ?
OR
Explain how a spectrum is formed.

Explanation

A band of coloured component of a light beam is called a spectrum.
When white light is incident on a prism , the rays corresponding to differnet colours bend through different angles on refraction.
Of the various colours in the visible region, red light bends the least and violet light bends th emost. Each colours emerges through the prism along a different path and becomes distinct. Hence, we get a spectrum of seven colours.