It is the phenomenon of splitting of a beam of white light into its constituent colors on passing through prism. The order of colors from the lower end are violet, indigo, blue, green, yellow, orange and red.

Cause of Dispersion
The refractive index of prism depends on wavelength according to cauchys relation namely

. Since different colors of light have different wavelength, their
m's are also different. Now red light has a longer wavelength than violet and so
mred <
mviolet. This means the
dv >
dr that is violet deviates by a larger angle than red light. All colors having wavelength is between red and violet deviate through intermediate angles leading to formation of a colored spectrum.