The ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is a constant for any two isotropic media bounded by a common interface. The refractive index n is given by: n = sin i/sin r, and n₁sin i = n₂sin r, where n₁ and n₂ are the refractive indices of the two media. The law also indicates that a P-wave incident upon a boundary will be reflected and refracted partly as a P-wave and partly as an S-wave. The law was formulated by the Dutch astronomer and mathematician Willebrord Snell (1591–1626).