birefringence n : splitting a ray into two parallel rays polarized perpendicularly [syn: double refraction]
Birefringence, or double refraction, is the decomposition of a ray of light into two rays (the ordinary ray and the extraordinary ray) when it passes through certain types of material, such as calcite crystals or boron nitride, depending on the polarization of the light. This effect can occur only if the structure of the material is anisotropic (directionally dependent). If the material has a single axis of anisotropy or optical axis, (i.e. it is uniaxial) birefringence can be formalized by assigning two different refractive indices to the material for different polarizations. The birefringence magnitude is then defined by
- \Delta n=n_e-n_o\,
The reason for birefringence is the fact that in anisotropic media the electric field vector \vec E and the dielectric displacement \vec D can be nonparallel (namely for the extraordinary polarisation), although being linearly related.
Birefringence can also arise in magnetic, not dielectric, materials, but substantial variations in magnetic permeability of materials are rare at optical frequencies.
Creating birefringenceWhile birefringence is often found naturally (especially in crystals), there are several ways to create it in optically isotropic materials.
- Birefringence results when isotropic materials are deformed such that the isotropy is lost in one direction (ie, stretched or bent). Example
- Applying an electric field can induce molecules to line up or behave asymmetrically, introducing anisotropy and resulting in birefringence. (see Pockels effect)
- Applying a magnetic field can cause a material to be circularly birefringent, with different indices of refraction for oppositely-handed circular polarizations (see Faraday effect).
Examples of uniaxial birefringent materialsMany plastics are birefringent, because their molecules are 'frozen' in a stretched conformation when the plastic is moulded or extruded. For example, cellophane is a cheap birefringent material, and Polaroid sheets are commonly used to examine for orientation in birefringent plastics like polystyrene and polycarbonate. Birefringent materials are used in many devices which manipulate the polarization of light, such as wave plates, polarizing prisms, and Lyot filters.
There are many birefringent crystals: birefringence was first described in calcite crystals by the Danish scientist Rasmus Bartholin in 1669.
Birefringence can be observed in amyloid plaque deposits such as are found in the brains of Alzheimer's victims. Modified proteins such as immunoglobulin light chains abnormally accumulate between cells, forming fibrils. Multiple folds of these fibers line up and take on a beta-pleated sheet conformation. Congo red dye intercalates between the folds and, when observed under polarized light, causes birefringence.
Cotton (Gossypium hirsutum) fiber is birefringent because of high levels of cellulosic material in the fiber's secondary cell wall.
Slight imperfections in optical fiber can cause birefringence, which can cause distortion in fiber-optic communication; see polarization mode dispersion.
Silicon carbide, also known as Moissanite, is strongly birefringent.
The refractive indices of several (uniaxial) birefringent materials are listed below (at wavelength ~ 590 nm)
Birefringence is often measured for rays propagating along one of the optical axes (or measured in a two-dimensional material). In this case, n has two eigenvalues which can be labeled n1 and n2. n can be diagonalized by:
where R(χ) is the rotation matrix through an angle χ. Rather than specifying the complete tensor n, we may now simply specify the magnitude of the birefringence Δn, and extinction angle χ, where Δn = n1 − n2.
- http://www.youtube.com/watch?v=BEClYQbuG7U Video of stress birefringence in Polymethylmethacrylate (PMMA or Plexiglas).
- Application note on the theory of birefringence
- Austine Wood Comarow: Paintings in Polarized Light, James Mann, Wasabi Publishing, 2005, ISBN 0-9768198-0-5.
birefringence in Bosnian: Birefringencija
birefringence in Czech: Dvojlom
birefringence in German: Doppelbrechung
birefringence in Estonian: Kaksikmurdumine
birefringence in Spanish: Birrefringencia
birefringence in French: Biréfringence
birefringence in Korean: 복굴절
birefringence in Italian: Birifrangenza
birefringence in Hebrew: שבירה כפולה
birefringence in Dutch: Dubbelbreking
birefringence in Japanese: 複屈折
birefringence in Low German: Dubbelbreken
birefringence in Polish: Dwójłomność
birefringence in Portuguese: Birrefringência
birefringence in Russian: Двойное лучепреломление
birefringence in Swedish: Dubbelbrytning
birefringence in Vietnamese: Khúc xạ đúp
birefringence in Ukrainian: Двозаломлення мінералів
birefringence in Chinese: 双折射