Property of anisotropic materials which possess different refractive indices according to the polarization of light passing through the material.
Birefringence is usually observed with the aid of a polarizing microscope.
When a plastic is stretched (molecular reorientation), and the refractive index parallel to the direction of stretching is altered so that it is no longer identical to that which is perpendicular to this direction, the material displays birefringence.
Also known as DOUBLE REFRACTION.