Snel's law of refraction

In optics, the rule that when a ray of light passes from one medium to another, the sine of the angle of incidence divided by the sine of the angle of refraction is equal to the ratio of the indices of refraction in the two media. For a ray passing from medium 1 to medium 2: n₂/n₁ = sin i/sin r where n₁ and n₂ are the refractive indices of the two media. The law was devised by the Dutch physicist, Willebrord Snel (1591-1626).