ABSTRACT
When a beam of ordinary unpolarized light is incident on a calcite or quartz crystal, there will be, in addition to the reflected beam, two refracted beams in place of the usual single one observed, for example, in glass. This phenomenon is called double refraction or birefringence. Upon measuring the angles of refraction for different angles of incidence, one finds that Snell's law of refraction holds for one ray but not for the other. The ray for which the law holds is called the ordinary ray and the other is called the extraordinary ray.
