Optic axis of a crystal

For the axis of an optical system, see Optical axis.

An optic axis of a crystal is a direction in which a ray of transmitted light suffers no birefringence (double refraction). An optic axis is a direction rather than a single line: all rays that are parallel to that direction exhibit the same lack of birefringence.

Crystals may have a single optic axis, in which case they are called uniaxial, or two optic axes, in which case they are biaxial. Non-crystalline (amorphous) materials generally exhibit no birefringence and thus have no optic axis. Uniaxial crystals belong to the tetragonal, hexagonal, or trigonal crystal systems (for example, calcite and quartz). In these crystals, the optic axis coincides with the crystallographic c-axis, and the refractive index is the same in all directions within the plane perpendicular to the optic axis, although the crystal as a whole remains optically anisotropic.

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