The law states that, for a given pair of media, the ratio of the sines of angle of incidence ( θ θ 1 ) and angle of refraction ( θ θ 2 ) is equal to the refractive index of the second medium with regard to the first ( n 21 ) which is equal to the ratio of the refractive indices ( n 2 n 1 ) of the two media, or equivalently, to the ratio of the phase velocities ( v 1 v 2 ) in the two media.