Optic axis of a crystal

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

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 propagates along the optical axis differently than in other directions. Light propagating along the optical 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 optical axis, then the beam is split into two rays (the ordinary and extraordinary) when passing through the crystal. These rays will be mutually perpendicularly 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 an uniaxial crystal. Crystals are classed according to the number of optic axes (uniaxial, biaxial). An uniaxial crystal is isotropic within the plane orthogonal to the optical 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 is variable and depends on the direction. Non-crystalline materials have no double refraction and thus, no optic axis. Some solid materials under specific conditions can demonstrate double refraction and optic axes.

Notes and references

1. ^ Hecht, Eugene (1987). Optics (2nd ed.). Addison Wesley. p. 283. ISBN 0-201-11609-X.