 Birefringence is the optical property of a material having the refractive index that depends on the polarization and propagation direction of a 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. Birefringent with non-cubic crystal structures are often birefringent, as are plastics under mechanical stress. Birefringence is responsible for the phenomenon of double-refraction whereby a ray of a light, when incident of on a birefringent material, is split by polarization into two rays taking slightly different paths. This effect was first described by the Danish scientist Rasmus Barfelin in 1669, who observed it in calcite, the crystal having one of the strongest birefringences. However it was not until the 19th century that Augustine Jean Framel described the phenomenon in terms of polarization, the understanding light as a wave with field components in transverse polarizations perpendicular to the direction of the wave vector.