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Snell's law:see refractionrefraction,
in physics, deflection of a wave on passing obliquely from one transparent medium into a second medium in which its speed is different, as the passage of a light ray from air into glass.
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a law governing the refraction of a beam of light at the boundary between two transparent media. It asserts that the ratio sin α /sin β is constant for the same two media; here, α is the angle of incidence of the beam on the boundary, and β is the angle of refraction.
The law was established by W. Snell around 1620 and by R. Descartes in 1637. The discovery of Snell’s law permitted the completion of the foundations of geometrical optics and made possible the formulation of Fermat’s principle. The law provided the basis for the introduction of the concept of the refractive index of a medium. In terms of refractive indexes, Snell’s law can be written in the form sin α/sin β = n2/n1 where n1 and n2 are the refractive indexes of the first and second media, respectively, through which the light travels. (See alsoREFRACTION OF LIGHT.)