Dispersion of White Light

A beam of white light passing through a prism is dispersed to form a visible spectrum. Red light is bent the least and violet bent the most, with other colors in between.

The speed of any single wavelength of light is the same in vacuum. But in glass, the red light travels fastest so it has the smallest index of refraction, and violet is the slowest so it has the largest index of refraction. After a beam of white light strikes a prism and refracted twice, the red light is bent the least and the violet light the most.