Birefringence

Birefringence is the optical property of anisotropic (or birefringent) bodies of splitting an incident ray into two refracted rays. One of the two refracted rays leaves the anisotropic body with a slight delay compared to the other. This delay depends on the difference between the refractive indices ng and np and the thickness of the body through which the rays pass. The difference ng - np is called the birefringence.

OPTICS: The property of certain media to double a light ray passing through them.