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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. So if the light would propagate along the optical axis of a uniaxial crystal (e.g. calcite, quartz), then nothing unusual happens. Light propagates along that axis with a speed independent of its polarization. However, if the light beam is not parallel to the optical axis, then, when passing through the crystal the beam is split into two rays: the ordinary and extraordinary, to be mutually perpendicular polarized.

The optic axis of a crystal is a direction rather a single line.^[1] If a ray in this direction suffers no birefringence, then every parallel ray suffers no birefringence too.

A crystal which has only one optic axis is called uniaxial crystal. An uniaxial crystal is isotropic within the plane orthogonal to the optical axis of the crystal. A crystal which has only two optic axes is called biaxial crystal and so on.

The refractive index of the ordinary ray is constant for any direction in the crystal, and the extraordinary ray variable and depends on the direction. In a uniaxial crystal for the direction parallel to the optical axis of the refractive indices are equal.

Non-crystalline materials have no double refraction. So they have no optic axis. However, some solid materials under some influences can get double refraction and optic axes.

[edit] Notes and references

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

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