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In some crystalline materials, like calcite and quartz, the speed of light is not the same in all directions. Such materials are characterized by two indices of refraction. So, they are often referred to as double-refracting or birefringent materials. When unpolarized light enters into the calcite crystal, the unpolarized light splits into two plane polarized rays that travel with different velocities, corresponding to two angles of refraction, now The two rays are polarized in two mutually perpendicular directions, as indicated by the dots and arrows. One ray, called the ordinary (O) ray, is characterized by an index of refraction that is the same in all directions. This means that if one could place a point source of light inside the crystal, the ordinary waves would spread out from the source as spheres
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The second plane-polarized ray, called the extraordinary (E) ray, travels with different speeds in different directions and hence is characterized by an index of refraction that varies with the direction of propagation.
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Waves and Optics