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Snell's law

(Snell-Descartes law, law of refraction)
(optical principle of refraction)

Snell's law (also known as Snell-Descartes law and the law of refraction) describes the path of light during refraction, i.e., the redirection of light as it passes through a boundary between different types of materials. The law relates the angle of incidence, i.e., the angle between the light's approach to the boundary and a line perpendicular to the boundary, to its angle of refraction, a similar angle regarding the direction of the light once it has traversed the boundary. The specific quality of the materials that determine the resulting refraction is called its refractive index. The law:

sin θ1   r2
—————— = ——
sin θ2   r1

(optics)
Further reading:
http://en.wikipedia.org/wiki/Snell's_law

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