| index of refraction | <physics> For a given wavelength, this is the ratio of the velocity of light in vacuum (c) to the velocity of light in a refractive material (for example, glass, plasma, etc.). (09 Oct 1997) |
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| law of refraction | For two given media, the sine of the angle of incidence bears a constant relation to the sine of the angle of refraction. Synonym: Descartes' law, Snell's law. (05 Mar 2000) |