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in optics, a relationship between the path taken by a ray of light in crossing the boundary or surface of separation between two contacting substances and the refractive index (q.v.) of each. This law was discovered in 1621 by the Dutch astronomer and mathematician Willebrord van Roijen Snell (1580-1626; also called Snellius). The account of Snell's law went unpublished until its mention by Christiaan Huygens in his treatise on light.