High Quality Content by WIKIPEDIA articles! 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. 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.Crystals with anisotropic crystal structures are often birefringent, as well as plastics under mechanical stress.