Birefringence

\_▐bī-ri-^▐frin-jәn(t)s\ n[ISV] (1898) (1) (double refraction) The difference between any two refractive indices in a single material. When the refractive indices measured along three mutually perpendicular axes are identical, the material is said to be optically isotropic. Orientation of a polymer by drawing may alter the refractive index in the direction of draw so that it is no longer equal to that in the perpendicular directions, in which event the material is said to display birefringence. Crystalline polymers, normally birefringent, may become optically isotropic at their melting points. Studies of birefringence provide useful information regarding the shapes of molecules, degrees of orientation and crystal habits. (2) Property of anisotropic materials, those which possess different refractive indices according to the direction of vibration of light passing through a crystal; usually observed with the aid of a polarizing microscope. The degree of birefringence is expressed...