An Etymological Dictionary of Astronomy and Astrophysics
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The relationship between angles of incidence and refraction for a wave incident on an interface between two media with different indices of refraction. The law states that the ratio of the sine of the → angle of incidence to the sine of the → angle of refraction is a constant: n₁/n₂ = sinθ₂/sinθ₁. See also → refractive index. Also known as Descartes' law or the law of refraction.

Named after Dutch mathematician Willebrord Snellius (1580-1626), one of the discoverers of the law; → law.