Snell's law 
(snĕlz)
(snĕlz)n.
A law describing the refraction of a ray of light at the surface between two media, such that the product of the refractive index of the first medium and the sine of the angle of incidence equals the product of the refractive index of the second medium and the sine of the angle of refraction.
[After Willebrord Snel van Royen (1580-1626), also called Willebrord Snellius, Dutch astronomer and mathematician who formulated the law.]
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