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(physics) The splitting of a ray of light into two parallel rays of perpendicular polarization by passage through an optically anisotropic medium; the property of a material that light passing through it is so split.

A calcite crystal laid upon a graph paper with blue lines showing the double refraction

(physics) The splitting of a ray of light into two parallel rays of perpendicular polarization by passage through an optically anisotropic medium; the property of a material that light passing through it is so split.

In this example, optic axis along the surface is shown perpendicular to plane of incidence. Incoming light in the s 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 p polarization (which becomes ordinary ray because "perpendicular polarisation" to optic axis) and so s polarization ray is undergoing greater refraction on entering and exiting the crystal.

(physics) The splitting of a ray of light into two parallel rays of perpendicular polarization by passage through an optically anisotropic medium; the property of a material that light passing through it is so split.

Doubly refracted image as seen through a calcite crystal, seen through a rotating polarizing filter illustrating the opposite polarization states of the two images.