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.
The optic axis of a crystal is a direction rather than a single line.[1] If a ray in this direction suffers no birefringence, neither will all parallel rays. A crystal with only one optic axis is called a uniaxial crystal. Crystals are classified according to the number of optic axes (uniaxial, biaxial) they have. A uniaxial crystal is isotropic within the plane orthogonal to the optic axis of the crystal.
The refractive index of the ordinary ray is constant for any direction in the crystal. The refractive index of the extraordinary ray varies depending on its direction. The ordinary passes undeviating through the crystal and show normal optical properties (reflection and refraction), but the extra-ordinary ray passes deviating through the crystal. Non-crystalline materials have no double refraction and thus, no optic axis. Some solid materials under specific conditions can demonstrate double refractions and optic axes.
Uniaxial and biaxial crystals
Crystals may exhibit a single optic axis, in which case they are uniaxial, or two different optic axes, in which case they are termed biaxial. Uniaxial and biaxial crystals are composed of an anisotropic, homogeneous medium which exhibits different electromagnetic properties in different directions. These crystals are generally non-magnetic; thus the material property that differs in different directions is the electric permittivity, rather than the magnetic permeability.