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Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light.〔(【引用サイトリンク】Olympus Microscopy Resource Center )〕 These optically anisotropic materials are said to be birefringent (or birefractive). The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with asymmetric 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 the Danish scientist Rasmus Bartholin in 1669, who observed it〔See: * Erasmus Bartholin, (''Experimenta crystalli islandici disdiaclastici quibus mira & infolita refractio detegitur'' ) (on birefringent Icelandic crystal through which is detected a remarkable and unique refraction ) (Copenhagen, Denmark: Daniel Paulli, 1669). * Erasmus Bartholin (January 1, 1670) ("An account of sundry experiments made and communicated by that learn'd mathematician, Dr. Erasmus Bartholin, upon a chrystal-like body, sent to him out of Island," ) ''Philosophical Transactions of the Royal Society of London'', 5 : 2041-2048.〕 in calcite, a crystal having one of the strongest birefringences. However it was not until the 19th century that Augustin-Jean Fresnel described the phenomenon in terms of polarization, understanding light as a wave with field components in transverse polarizations (perpendicular to the direction of the wave vector). ==Explanation== A mathematical description of wave propagation in a birefringent medium is presented below. Following is a qualitative explanation of the phenomenon 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「birefringence」の詳細全文を読む スポンサード リンク
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