For light, refraction follows Snell's law, which states that, for a given pair of media, the ratio of the sines of the angle of incidence θ θ 1 and angle of refraction θ θ 2 is equal to the ratio of phase velocities v 1 v 2 in the two media, or equivalently, to the refractive indices n 2 n 1 of the two media: