Anisotropic crystals have crystallographically distinct axes and interact with light in a manner that is dependent upon the orientation of the crystalline lattice with respect to the incident light. When light enters a non-equivalent axis in a anisotropic crystal, it is refracted into two rays each polarised with the vibration directions oriented at right angles to one another, and traveling at different velocities. This phenomenon is termed double- or bi-refraction (or birefringence) and is seen to a greater or lesser degree in all anisotropic crystals.