Snells law of Refraction
Snell's law states that the ratio of the Sines of the angles of incidence and refraction is equivalent to the ratio of velocity of light in the two media, or equivalent to the reciprocal
of the ratio of the indices of refraction:
as the angle measured from the normal of the boundary,
as the velocity of light in the respective medium and
as the refractive index of the respective medium.