You must have heard the terms dispersion and spectrum. For example, in easy words the RAINBOW pattern is called spectrum and this happens due to dispersion. Let us discuss these two in detail:

When white light after passing through a prism falls on a screen, only the seven colors (VIBGYOR) are mainly seen. These band of colors obtained on screen, is called spectrum. The prism splits the colors present in white light. The splitting of white light into its constituent colors is called dispersion .

Cause of Dispersion and spectrum:

When white light is incident on the first surface of the prism and enters it, light of different color is refracted (or bended or deviated) through different angles. Therefore the dispersion of white light into its constituent colors takes place at the first surface of prism. This happens because of the refractive index of the medium. More the refractive index, more will be the deviation.

The deviation towards the base of prism is different for  light of different colors because of their different speeds in glass (because refractive index n= c/v). The  red color is deviated the least , while the violet color is deviated the most. On the second surface of the prism, only refraction takes place (from air to glass) and different colours are deviated through different angles. Here also violet is deviated the most and the red the least. As a result,

the colors get further separated on refraction at second surface. The light coming out of the prism, therefore, has different colors that spread out to form a spectrum.