birefringence

right|thumb|A calcite crystal laid upon a graph paper with blue lines showing the double refraction right|thumb|In this example, optic axis along the surface is shown perpendicular to plane of incidence. Incoming light in the polarization (which means perpendicular to plane of incidence – and so in this example becomes "parallel polarisation" to optic axis, thus is called extraordinary ray) sees a greater refractive index than light in the polarization (which becomes ordinary ray because "perpendicular polarisation" to optic axis) and so polarization ray is undergoing greater refraction on entering and exiting the crystal.

Birefringence, also called double refraction, is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefringent or birefractive. The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with non-cubic crystal structures are often birefringent, as are plastics under mechanical stress.