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An introduction to minerals and rocks under the microscope

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Figure 29 (a) Side-view through a calcite cleavage rhomb, showing how light is doubly refracted into two rays as it passes through the crystal. For calcite, the refractive indices (or speeds) of the two rays are very different, so they travel along quite different paths through the crystal. (b) Double refraction in natural calcite (Iceland spar). The difference in the speeds (i.e. refractive indices) of the two rays is so great that the eye perceives two images.

 2.2.2 Double refraction