Optic axis of a crystal
In crystal optics, the optical axis indicates (often also referred to as C-axis) the direction in optically anisotropic ( birefringent ) crystalline, along which each polarization component of a light ray experiences the same refractive index.
Most of the crystals are optically anisotropic. In such crystals, the refractive index of the polarization and direction of light propagation depends.
In uniaxial crystals, there is an optical axis that lies in the direction of main refractive index unikalen. A beam of light along the optical axis behaves in an isotropic crystal.
In biaxial crystals, there are two optical axes. They lie in the plane that span the vectors of the smallest and the largest of the three principal refractive indices ( principal refractive axes). An optical axis can be transferred by reflection in one of these two principal refractive axes in the other optical axis. In biaxial crystal resulting from a light beam passing along the optical axis for polarization components in the direction of the middle of the three main refractive axes an ordinary ray and all other polarization components an extraordinary ray having a different direction of propagation for each polarization component. Due to the same refractive indices of all polarization components all polarization directions are equal and there is no discrete split into two beams instead. Instead, it comes to the conical refraction of the extraordinary ray. The means for non-polarized light that can be seen a beam cone, which contains the optical axis in its lateral surface.