Birefringence

A calcite crystal laid upon a graph paper with blue lines showing the double refraction
In this example, optic axis along the surface is shown perpendicular to plane of incidence. Incoming light in the s polarization (which means perpendicular to plane of incidence – and so in this example becomes "parallel polarisation" to optic axis, thus is called extraordinary ray) sees a greater refractive index than light in the p polarization (which becomes ordinary ray because "perpendicular polarisation" to optic axis) and so s polarization ray is undergoing greater refraction on entering and exiting the crystal.

Birefringence means double refraction. It is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light.[1] These optically anisotropic materials are described as birefringent or birefractive. The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with non-cubic crystal structures are often birefringent, as are plastics under mechanical stress.

Birefringence is responsible for the phenomenon of double refraction whereby a ray of light, when incident upon a birefringent material, is split by polarization into two rays taking slightly different paths. This effect was first described by Danish scientist Rasmus Bartholin in 1669, who observed it[2] in Iceland spar (calcite) crystals which have one of the strongest birefringences. In the 19th century Augustin-Jean Fresnel described the phenomenon in terms of polarization, understanding light as a wave with field components in transverse polarization (perpendicular to the direction of the wave vector).[3][4]

  1. ^ Abramowitz, Mortimer; Davidson, Michael W. "Olympus Microscopy Resource Center". Olympus Life Science Inc. Retrieved 2021-07-21.
  2. ^ See:
  3. ^ A. Fresnel, "Note sur le calcul des teintes que la polarisation développe dans les lames cristallisées" et seq., Annales de Chimie et de Physique, Ser. 2, vol. 17, pp. 102–111 (May 1821), 167–96 (June 1821), 312–15 ("Postscript", July 1821); reprinted (with added section nos.) in Fresnel, 1866–70, vol. 1, pp. 609–648; translated as "On the calculation of the tints that polarization develops in crystalline plates, & postscript", Zenodo: 4058004 (Creative Commons), 2021; §14.
  4. ^ A. Fresnel, "Extrait d'un Mémoire sur la double réfraction", Annales de Chimie et de Physique, Ser. 2, vol. 28, pp. 263–279 (March 1825); reprinted as "Extrait du second Mémoire sur la double réfraction" in Fresnel, 1866–70, vol. 2, pp. 465–478; translated as "Extract of a [second] memoir on double refraction", Zenodo: 5442206, 2021 (open access).