doublerefraction

Double refraction, also known as birefringence, is an optical property in certain materials where a ray of light splits into two refracted rays. This phenomenon occurs in anisotropic crystals, meaning their optical properties vary with the direction of light propagation and polarization. When unpolarized light enters a birefringent material, it is separated into two polarized beams. These beams are called the ordinary ray (o-ray) and the extraordinary ray (e-ray). The o-ray travels at the same speed regardless of its polarization direction, while the e-ray's speed, and thus its refractive index, depends on its polarization orientation relative to the crystal's optical axis.

The two rays generally travel in different directions and are polarized perpendicularly to each other. The amount of splitting depends on the material's birefringence, which is the difference between the refractive indices of the o-ray and the e-ray. Materials like calcite, quartz, and mica are common examples of birefringent substances. This property has numerous applications in optics, including in polarizing filters, optical modulators, and microscopes. For instance, polarizers are often made from birefringent materials to selectively transmit light polarized in a specific direction. The study of birefringence has been crucial in understanding the wave nature of light and the optical properties of crystalline structures.