Birefringence is the property of showing more than one refractive index as a function of particle orientation and wavelength. Such a particle will exhibit interference colors when viewed between crossed circular polarized filters. They will typically show extinction positions (see below) with rotation of the stage when viewed between crossed linear polarizing filters.

Anomalous birefringence is the result of birefringence varying by wavelength. The result is anomalous interference colors. Silicon carbide and crocidolite asbestos are two common examples. Crocidolite has higher birefringence in red light (longer Wavelengths) than in blue. As a result, very thin fibers of crocidolite appear red between crossed polarizing filters. Thicker fibers appear blue because of the strong blue color of the mineral.

Silicon carbide has higher birefringence in blue light (Shorter Wavelenghts) than in red. As a result, blue wavelengths cycle more rapidly than red wavelengths. Yellow interference color begins for thinner particles and first order red appears purple because blue is increasing well before red significantly decreases. This effect changes the color sequence through the whole range of microscopic silicon carbide particles.

When a material is placed under stress the distribution of the electrons in the material is changed. The amount of change is different for each material and is a characteristic of the material. The photoelastic constant of the matrial is a measure of the electron displacement (strain) as a function of the load (stress) applied as long as the deformation is elastic, springs back when the load is removed. If the Young's Modulus of the matrial is exceeded, then some of the deformation becomes permenant. In some materials the applied load can be "frozen" in place, as in the case of high stress glass sheet. Polarized light can make the displacement visible. Both plastic deformation and elastic deformation result in an anisotropic distribution of electrons in the material that becomes visible as interference colors when the object is viewed between crossed linear or crossed circular polarizing filters. Click on the photographs below for more information.

Polarized light is depolarized at the interface between a conductive particle and a non-conductive mounting medium. This light halo effect with transmitted crossed polarized light indicates an opaque particle is a wear metal particle or at least is conductive. Graphite is sufficiently conductive to produce this effect. Pencil debris can be distinguished from combustion residue by this effect.

If the refractive index of a transparent particle is much different than the medium in contact with it, then the polarized beam can be changed at the interface as a result of reflection. If the interface is aligned with the polarizer or analyzer then the beam is not affected. In other orientations reflection at the interface results in rotation of the polarized beam and the interface appears to show a first order white interference color.