Light is an electromagnetic vibration, and can be likened to a transfer of energy by vibrating ‘particles’ along a path from the source to the receiver. In a ray of light of a single wavelength, the wave is generated by the vibration of ‘particles’ lying along the path of the ray. When, however, a ray of light passes through an anisotropic mineral plate, the light is split into two unequal components which vibrate in mutually perpendicular directions, and which are related to the refractive indices of the mineral in those directions.