Difference between revisions of "Birefringence" - New World Encyclopedia
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A common feature of optical microscopes is a pair of crossed [[polarizer|polarizing]] filters. Between the crossed polarizers, a birefringent sample will appear bright against a dark (isotropic) background. | A common feature of optical microscopes is a pair of crossed [[polarizer|polarizing]] filters. Between the crossed polarizers, a birefringent sample will appear bright against a dark (isotropic) background. | ||
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+ | ==Elastic birefringence== | ||
+ | Another form of birefringence is observed in anisotropic [[elastic deformation|elastic]] materials. In these materials, [[S-wave|shear wave]]s split according to similar principles as the light waves discussed above. The study of birefringent shear waves in the earth is a part of [[seismology]]. Birefringence is also used in optical mineralogy to determine the chemical composition, and history of minerals and rocks. | ||
== Applications of birefringence == | == Applications of birefringence == | ||
Birefringence is widely used in optical devices, such as [[liquid crystal display]]s, [[electro-optic modulator|light modulators]], [[Lyot filter|color filters]], [[wave plate]]s, [[optical axis gratings]], etc. It also plays important role in [[second harmonic generation]] and many other [[Nonlinear optics|nonlinear processes]]. It is also utilized in medical diagnostics. Needle [[biopsies|biopsy]] of suspected [[gout]]y joints will be negatively birefringent if [[urate]] crystals are present. | Birefringence is widely used in optical devices, such as [[liquid crystal display]]s, [[electro-optic modulator|light modulators]], [[Lyot filter|color filters]], [[wave plate]]s, [[optical axis gratings]], etc. It also plays important role in [[second harmonic generation]] and many other [[Nonlinear optics|nonlinear processes]]. It is also utilized in medical diagnostics. Needle [[biopsies|biopsy]] of suspected [[gout]]y joints will be negatively birefringent if [[urate]] crystals are present. | ||
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== See also == | == See also == | ||
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* [[Light]] | * [[Light]] | ||
* [[Refraction]] | * [[Refraction]] | ||
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==References== | ==References== | ||
* Born, Max, and Emil Wolf. 1999. ''Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light''. 7th ed. Cambridge, UK: Cambridge University Press. ISBN 0521642221. | * Born, Max, and Emil Wolf. 1999. ''Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light''. 7th ed. Cambridge, UK: Cambridge University Press. ISBN 0521642221. | ||
+ | |||
+ | * Halliday, David, Robert Resnick, and Kenneth S. Krane. 2001. ''Physics''. Vol. 2. 5th ed. New York: John Wiley. ISBN 0471401943. | ||
* Sharma, Kailash K. 2006. ''Optics: Principles and Applications''. Burlington, MA: Academic Press. ISBN 0123706114. | * Sharma, Kailash K. 2006. ''Optics: Principles and Applications''. Burlington, MA: Academic Press. ISBN 0123706114. | ||
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* Elert, Glenn. 2007. [http://hypertextbook.com/physics/waves/refraction/ The Physics Hypertextbook: Refraction] ''hypertextbook.com''. Retrieved April 19, 2007. (Gives lists of several birefringent/trirefringent materials.) | * Elert, Glenn. 2007. [http://hypertextbook.com/physics/waves/refraction/ The Physics Hypertextbook: Refraction] ''hypertextbook.com''. Retrieved April 19, 2007. (Gives lists of several birefringent/trirefringent materials.) | ||
+ | ==External links== | ||
− | + | * [http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/biref.html Birefrigent Materials.] ''Department of Physics and Astronomy, Georgia State University''. Retrieved April 19, 2007. | |
+ | |||
+ | * [http://scienceworld.wolfram.com/physics/Birefringence.html Birefringence.] ''Eric Weisstein's World of Physics''. Retrieved April 19, 2007. | ||
[[Category:Physical sciences]] | [[Category:Physical sciences]] |
Revision as of 21:57, 19 April 2007
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, depending on the polarization of the light. This effect can occur only if the structure of the material is anisotropic. If the material has a single axis of anisotropy, (i.e. it is uniaxial) birefringence can be formalised by assigning two different refractive indices to the material for different polarizations. The birefringence magnitude is then defined by:
where no and ne are the refractive indices for polarizations perpendicular (ordinary) and parallel (extraordinary) to the axis of anisotropy respectively.
Birefringence can also arise in magnetic, not dielectric, materials, but substantial variations in magnetic permeability of materials are rare at optical frequencies.
Creating birefringence
While 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 birefringent materials
Many 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. 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), from [1].
Material | no | ne | Δn |
beryl Be3Al2(SiO3)6 | 1.602 | 1.557 | -0.045 |
calcite CaCO3 | 1.658 | 1.486 | -0.172 |
calomel Hg2Cl2 | 1.973 | 2.656 | +0.683 |
ice H2O | 1.309 | 1.313 | +0.014 |
lithium niobate LiNbO3 | 2.272 | 2.187 | -0.085 |
magnesium fluoride MgF2 | 1.380 | 1.385 | +0.006 |
quartz SiO2 | 1.544 | 1.553 | +0.009 |
ruby Al2O3 | 1.770 | 1.762 | -0.008 |
rutile TiO2 | 2.616 | 2.903 | +0.287 |
peridot (Mg, Fe)2SiO4 | 1.690 | 1.654 | -0.036 |
sapphire Al2O3 | 1.768 | 1.760 | -0.008 |
sodium nitrate NaNO3 | 1.587 | 1.336 | -0.251 |
tourmaline (complex silicate ) | 1.669 | 1.638 | -0.031 |
zircon, high ZrSiO4 | 1.960 | 2.015 | +0.055 |
zircon, low ZrSiO4 | 1.920 | 1.967 | +0.047 |
Biaxial birefringence
Biaxial birefringence, also known as trirefringence, describes an anisotropic material that has more than one axis of anisotropy. For such a material, the refractive index tensor n, will in general have three distinct eigenvalues that can be labelled nα, nβ and nγ.
The refractive indices of some trirefringent materials are listed below (at wavelength ~ 590 nm), from [2].
Material | nα | nβ | nγ |
borax | 1.447 | 1.469 | 1.472 |
epsom salt MgSO4·7(H2O) | 1.433 | 1.455 | 1.461 |
mica, biotite | 1.595 | 1.640 | 1.640 |
mica, muscovite | 1.563 | 1.596 | 1.601 |
olivine (Mg, Fe)2SiO4 | 1.640 | 1.660 | 1.680 |
perovskite CaTiO3 | 2.300 | 2.340 | 2.380 |
topaz | 1.618 | 1.620 | 1.627 |
ulexite | 1.490 | 1.510 | 1.520 |
Measuring birefringence
Birefringence and related optical effects (such as optical rotation and linear or circular dichroism) can be measured by measuring the changes in the polarization of light passing through the material. These measurements are known as polarimetry.
A common feature of optical microscopes is a pair of crossed polarizing filters. Between the crossed polarizers, a birefringent sample will appear bright against a dark (isotropic) background.
Elastic birefringence
Another form of birefringence is observed in anisotropic elastic materials. In these materials, shear waves split according to similar principles as the light waves discussed above. The study of birefringent shear waves in the earth is a part of seismology. Birefringence is also used in optical mineralogy to determine the chemical composition, and history of minerals and rocks.
Applications of birefringence
Birefringence is widely used in optical devices, such as liquid crystal displays, light modulators, color filters, wave plates, optical axis gratings, etc. It also plays important role in second harmonic generation and many other nonlinear processes. It is also utilized in medical diagnostics. Needle biopsy of suspected gouty joints will be negatively birefringent if urate crystals are present.
See also
- Crystal optics
- John Kerr
- Light
- Refraction
ReferencesISBN links support NWE through referral fees
- Born, Max, and Emil Wolf. 1999. Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light. 7th ed. Cambridge, UK: Cambridge University Press. ISBN 0521642221.
- Halliday, David, Robert Resnick, and Kenneth S. Krane. 2001. Physics. Vol. 2. 5th ed. New York: John Wiley. ISBN 0471401943.
- Sharma, Kailash K. 2006. Optics: Principles and Applications. Burlington, MA: Academic Press. ISBN 0123706114.
- Elert, Glenn. 2007. The Physics Hypertextbook: Refraction hypertextbook.com. Retrieved April 19, 2007. (Gives lists of several birefringent/trirefringent materials.)
External links
- Birefrigent Materials. Department of Physics and Astronomy, Georgia State University. Retrieved April 19, 2007.
- Birefringence. Eric Weisstein's World of Physics. Retrieved April 19, 2007.
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