The property, possessed by certain crystals (notably calcite), of forming two refracted rays from a single incident ray. The ordinary ray obeys the normal laws of refraction. The other refracted ray, called the extraordinary ray, follows different laws. The light in the ordinary ray is polarized at right angles to the light in the extraordinary ray. Along an optic axis the ordinary and extraordinary rays travel with the same speed. Some crystals, such as calcite, quartz, and tourmaline, have only one optic axis; they are uniaxial crystals. Others, such as mica and selenite, have two optic axes; they are biaxial crystals. The phenomenon is also known as birefringence and the double-refracting crystal as a birefringent crystal. See also polarization.
Subjects: Chemistry — Physics.