"Birefringence"

Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light.[1] These optically anisotropic materials are said to be birefringent. The birefringence is often quantified by the maximum difference in refractive index within the material. Birefringence is also often used as a synonym for double refraction, the decomposition of a ray of light into two rays when it passes through a birefringent material. This effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite.[2] Crystals with anisotropic crystal structures are often birefringent, as well as plastics under mechanical stress.