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Photonics Dictionary

double refraction

The separation of unpolarized light into two plane-polarized elements by a doubly refracting crystal. When a crystal, such as calcite, is placed between the eye and a pinhole in a card, two bright dots may be seen. If the crystal is then rotated around the line of sight, one dot moves in a circle around the other, which remains stationary. This shows that there must be two refracted rays that reveal a difference in refractive index; they form an angle of about 6º 9' when the light is normally incident on the natural crystal face. The stationary, or "ordinary" ray, reacts to a greater index than does the other, the "extraordinary" ray. Huygens explained these phenomena with respect to the fact that the ordinary wave has a spherical wavefront, moving with the same speed in all directions, whereas the extraordinary wave has either a maximum or minimum speed along the optic axis.
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