If white light is passed through a triangular prism (a glass or plastic block), it is split into a spectrum of different colours. This is called dispersion. The refractive index of the glass or plastic is slightly different for each colour of light, so each colour is refracted by a different amount. The light rays are bent or refracted twice: once as they enter the prism, and again in the same direction as they leave it. The dispersion increases each time, which is why the colours are separated so much. Red is always bent least. And violet is bent most.