  right|thumb|200px|doubly refracted image as seen through a calcite crystal, seen through a rotating polarizing filter illustrating the opposite polarization states of the two images. birefringence 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 said to be birefringent (or birefractive). the birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. crystals with asymmetric crystal structures are often birefringent, as well as plastics under mechanical stress. birefringence is responsible for the phenomenon of double refraction whereby a ray of light, when incident upon a birefringent material, is split by polarization into two rays taking slightly different paths. this effect was first described by the danish scientist rasmus bartholin in 1669, who observed iterasmus bartholin, experimenta crystalli islandici disdiaclastici quibus mira & infolita refractio detegitur [experiments on birefringent icelandic crystal through which is detected a remarkable and unique refraction] (copenhagen, denmark: daniel paulli, 1669). see also: erasmus bartholin (january 1, 1670) "an account of sundry experiments made and communicated by that learn'd mathematician, dr. erasmus bartholin, upon a chrystal-like body, sent to him out of island," philosophical transactions of the royal society of london, vol. 5, pages 2039-2048. in calcite, a crystal having one of the strongest birefringences. however it was not until the 19th century that augustin-jean fresnel described the phenomenon in terms of polarization, understanding light as a wave with field components in transverse polarizations (perpendicular to the direction of the wave vector). ==explanation== the simplest (and most common) type of birefringence is described as uniaxial, meaning that there is a single direction governing the optical anisotropy whereas all directions perpendicular to it (or at a given angle to it) are optically equivalent. thus rotating the material around this axis does not change its optical behavior. this special direction is known as
Optical_mineralogy Polarization_(waves)