Double refraction, also called birefringence,  an optical property in which a single ray of unpolarized light entering an anisotropic medium is split into two rays, each traveling in a different direction. One ray (called the extraordinary ray) is bent, or refracted, at an angle as it travels through the medium; the other ray (called the ordinary ray) passes through the medium unchanged.
						

							
the separation of a ray of light into two unequally refracted, plane-polarized rays of orthogonal polarizations, occurring in crystals in which the velocity of light rays is not the same in all directions. Some crystals in nature have different index of refraction in different directions, and the index of refraction depends on the direction of polarization of the light entering the crystal. An example for such crystal is Calcite, which is also called: Iceland Spar. Since the index of refraction determines the speed of light in the medium (c/n), thus different indexes of refraction for different polarizations creates phase difference between different polarization. Thus, the material split the light incident on it into two separate polarized beams, at right angle to each other.