Double Refraction (#74-75)
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Light entering minerals with low crystal symmetry is broken into two rays. Calcite, for example, shows a doubling of images viewed through it. The two rays are polarized so that the light in each vibrates in a direction at right angles to the other. The same polarization happens in the mineral mica, but in mica one of the rays is completely absorbed in one direction. This effect is called double refraction. In the display, strips of mica appropriately positioned over calcite illustrate the combined effects of polarization and double refraction.
74. Calcite (Double Refraction)
75. Calcite & Mica (Double Refraction & Polarization)