White Light

A prism can be used to split a beam of white light into colours of the spectrum.This is called dispersion.Each colour is refracted by a different amountViolet light travels more slowly.n_{violet light} > n_{red light} Dispersion happens because the refractive index of glass depends on the wavelength of the light. Violet light has the shortest wavelength and highest frequency, so it is refracted most; red light has the longest wavelength and is refracted least. In general, longer wavelengths undergo a smaller refraction than shorter wavelengths.At each surface Snell's law applies:n₁ sinθ₁ = n₂ sinθ₂The amount of bending depends on the angle of incidence and on the ratio of the refractive indices of the two media. Since n = c / c_s, violet light, with the larger refractive index, travels more slowly in glass than red light.When light refracts, its speed and wavelength change but its frequency stays the same.In a triangular prism, each colour leaves at a different angle, so the colours spread out into a spectrum. In a parallel-sided glass block, the colours leave parallel to each other and are only slightly displaced sideways.