Optical axis (crystal optics)

In crystal optics , the optical axis (often also called the c-axis ) denotes the direction in an optically anisotropic ( birefringent ) crystal along which every polarization component of a light beam experiences the same refractive index . It is not to be confused with the optical axis of optical systems .

Most crystals are optically anisotropic and therefore have (at least) one optical axis. In such crystals, the refractive index depends on the polarization and the direction of propagation of the light.

In uniaxial crystals, i.e. H. In the whirling crystal systems ( trigonal , tetragonal and hexagonal ), there is an optical axis that lies in the direction of the unique main refractive index . A light beam along the optical axis behaves like in an isotropic crystal.

In biaxial crystals, i.e. H. In the orthorhombic , monoclinic and triclinic crystal systems , there are two optical axes. They lie in the plane that is spanned by the vectors of the smallest and largest of the three main refractive indices (main refractive axes). An optical axis can be converted into the other optical axis by mirroring it on one of these two main refraction axes.

In the biaxial crystal, a light beam that runs along one of the two optical axes creates a proper beam for polarization components in the direction of the central main refractive axis . For all other polarization components an extraordinary beam is created, which has a different direction of propagation for each polarization component but the same refractive index. Therefore, all polarization directions are equal, there is no discrete splitting into two beams. Instead, the extraordinary ray is refracted conically . For unpolarized light, this means that you see a beam cone that contains the optical axis in its lateral surface.