Optic axis of a crystal

The optic axis of a crystal is the direction in which a ray of transmitted light suffers no birefringence (double refraction). Due to the internal structure of the crystal (the specific structure of the crystal lattice, the form of atoms or molecules of its components), light behaves differently when propagating along the optic axis than in other directions. Light propagating along the optic axis of a uniaxial crystal (e.g. calcite, quartz), has no unusual results. Light propagates along that axis with a speed independent of its polarization. If the light beam is not parallel to the optic axis, then the beam is split into two rays (the ordinary and extraordinary) when passing through the crystal. These rays will be mutually orthogonally polarized.