birefringente

Birefringente, commonly referred to as birefringence, is the optical property of certain anisotropic materials to split a beam of light into two rays with different velocities and polarizations as it passes through. In anisotropic crystals, the refractive index depends on both the direction of light and its polarization. The two rays are known as the ordinary ray (o-ray) and the extraordinary ray (e-ray). In uniaxial crystals there is one optic axis; the o-ray experiences a constant index, while the e-ray’s index varies with the propagation direction relative to the optic axis. In biaxial crystals there are two optic axes and a more complex index surface.

When a birefringent material is placed between crossed polarizers or observed with a polarizing microscope, interference colors arise from the phase retardation between the o- and e-rays. The retardation is Δ = 2π t (n_e − n_o)/λ, where t is thickness and λ is the vacuum wavelength. The phenomenon underpins devices that control polarization, such as waveplates (quarter-wave and half-wave plates) and various optical modulators, and it is used in polarimetry and stress analysis (photoelasticity).

Birefringence is common in many transparent crystals and polymers. Calcite is among the strongest natural birefringent materials; quartz exhibits birefringence at a lower level, while many polymers show significant effects. The term birefringente is used in several languages as the adjective form of birefringence.