Birefringence in a calcite crystal. Also called double refraction, this phenomenon occurs where a material is not uniform and affects light differently in different directions (anisotropy). This phenomenon, which is similar to polarisation, splits the light ray into two components, the fast or ordinary ray and the slow or extraordinary ray. Because the two types of waves travel at different speeds, they get out of phase, producing the double images of the line and the word behind the crystal. Naturally birefringent materials are boron nitride and calcite.