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double refraction

Double refraction in Iceland Spar
Double refraction displayed by a crystal of Iceland spar
Double refraction, also called birefringence, is the splitting of a light wave into two waves, traveling at different speeds, by an optically anisotropic medium such as a crystal of Iceland spar (calcite), quartz, or topaz. In the phenomenon of double refraction a ray of unpolarized light is split into two rays plane-polarized (see polarization) at right-angles to each other. One is refracted in the normal way, the other with a refractive index that depends on the original direction of the ray.

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