birefringentit

Birefringentit, or birefringence, describes the property of anisotropic materials to have different refractive indices depending on the polarization and direction of light. When such light enters the material, it is split into two rays that propagate at different speeds: the ordinary ray, with refractive index n_o, and the extraordinary ray, with index n_e. The two rays emerge with a relative phase shift, often producing observable interference effects. The magnitude of birefringence is defined as Δn = n_e − n_o and is strongest in crystals with low symmetry.

In uniaxial crystals, there is one optic axis. Light polarized perpendicular to the optic axis experiences n_o, while light polarized within the ordinary plane experiences an angle-dependent n_e. In biaxial crystals, two optic axes exist and three principal indices n_alpha, n_beta, n_gamma govern the propagation. Birefringence depends on wavelength (dispersion) and is typically larger in calcite and quartz than in many polymers.

Common birefringent materials include calcite, tourmaline, quartz, and lithium niobate, as well as artificially stressed polymers and liquid crystals. Practical effects are exploited in optical devices and measurements: wave plates (half- and quarter-wave plates) introduce controlled phase shifts; polarizers and analyzers use birefringence to modulate polarization; and in photoelasticity, birefringence reveals internal stresses in transparent components.

Observation and measurement are typically performed with polarized light, such as in polarized light microscopy, where birefringence produces interference colors that help identify minerals and characterize materials.