Birefringence

All minerals that do not crystallize in the cubic crystal system are optically anisotropic, meaning that the optical properties of these minerals depend on the direction in which the light travels through the crystal.

When a mineral is illuminated by a light source (e.g., the sun), some of the light is reflected at the surface and some is refracted into the mineral. In anisotropic minerals, the refracted light ray is then split into two partial rays with different speeds. This is due to the fact that a light ray is an electromagnetic wave. When it encounters a mineral, the electric field (E) of this wave interacts with the electron clouds of the atoms in the crystal lattice. The electric field displaces the electrons relative to the atomic nuclei, creating microscopic dipoles, a phenomenon known as dielectric polarization . The ease with which these electron clouds can be displaced is described by the permittivity . In a cubic crystal, the bond strength and the atomic distances are identical in all three spatial dimensions. The electrons can be displaced equally easily in every direction, which is why the permittivity is a simple number (scalar). In anisotropic minerals, the chemical bonds and atomic distances are direction-dependent. The permittivity here is no longer a scalar, but a second-rank tensor. This results in two partial rays with different velocities, with the faster ray being slowed down and thus deflected less (minimal refractive index). The slower ray is slowed down/deflected more (maximum refractive index). The value of the birefringence is the difference between the maximum and minimum refractive indices (δ = n_{_max} - n_{_min}) and is material-specific.

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ZEISS Microscopy from Germany, Michel-Lévy interference color chart (21257606712) , Reduced by TerraCollect, CC BY-SA 2.0