Optic axis of a crystal
Encyclopedia
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. 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.
Ray (optics)
In optics, a ray is an idealized narrow beam of light. Rays are used to model the propagation of light through an optical system, by dividing the real light field up into discrete rays that can be computationally propagated through the system by the techniques of ray tracing. This allows even very...
of transmitted light suffers no birefringence
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
(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
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
, quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of 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. 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.