1. The band of coloured components of a beam of light is called a spectrum.
2. The refractive index of the material of the prism is different for different colours.
3. White light is composed of seven colours. When white light is incident on a prism, the constituent colours get refracted through different angles and get dispersed.
4. So, the rays of each colour emerge along different paths and become distinct. Hence, we get a spectrum of seven different colours.