Snell 's law states that the ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of phase velocities in the two media , or equivalent to the reciprocal of the ratio of the indices of refraction : with each as the angle measured from the normal of the boundary , as the velocity of light in the respective medium ( SI units are meters per second , or m\/s ) and as the refractive index ( which is unitless ) of the respective medium .
The law states that when light passes through different materials ( for example from air to glass ) the ratio of sines of the incidence ( incoming ) angle and the refraction ( outgoing ) angle does not change : with each as the angle measured from the normal of the boundary , as the velocity of light in the respective medium ( SI units are meters per second , or m\/s ) and as the refractive index ( which has no units ) of the medium .