Editing Polarization (waves) (section)

== Measurement techniques involving polarization ==
Some optical measurement techniques are based on polarization. In many other optical techniques polarization is crucial or at least must be taken into account and controlled; such examples are too numerous to mention.

=== Measurement of stress ===
[[File:Polarized Stress Glasses.jpg|thumb|Stress in plastic glasses]]
In [[engineering]], the phenomenon of [[birefringence#Stress-induced birefringence|stress induced birefringence]] allows for stresses in transparent materials to be readily observed. As noted above and seen in the accompanying photograph, the chromaticity of birefringence typically creates colored patterns when viewed in between two polarizers. As external forces are applied, internal stress induced in the material is thereby observed. Additionally, birefringence is frequently observed due to stresses "frozen in" at the time of manufacture. This is famously observed in [[cellophane]] tape whose birefringence is due to the stretching of the material during the manufacturing process.

=== Ellipsometry ===
{{Main|Ellipsometry}}
Ellipsometry is a powerful technique for the measurement of the optical properties of a uniform surface. It involves measuring the polarization state of light following specular reflection from such a surface. This is typically done as a function of incidence angle or wavelength (or both). Since ellipsometry relies on reflection, it is not required for the sample to be transparent to light or for its back side to be accessible.

Ellipsometry can be used to model the (complex) refractive index of a surface of a bulk material. It is also very useful in determining parameters of one or more [[thin film]] layers deposited on a substrate. Due to their [[Thin-film interference|reflection properties]], not only are the predicted magnitude of the ''p'' and ''s'' polarization components, but their relative phase shifts upon reflection, compared to measurements using an ellipsometer. A normal ellipsometer does not measure the actual reflection coefficient (which requires careful photometric calibration of the illuminating beam) but the ratio of the ''p'' and ''s'' reflections, as well as change of polarization ellipticity (hence the name) induced upon reflection by the surface being studied. In addition to use in science and research, ellipsometers are used [[ellipsometry#In situ ellipsometry|in situ]] to control production processes for instance.<ref name="GoldsteinGoldstein2011">{{cite book|author1=Dennis Goldstein|author2=Dennis H. Goldstein|title=Polarized Light, Revised and Expanded|date=3 January 2011|publisher=CRC Press|isbn=978-0-203-91158-7}}</ref>{{rp|585ff}}<ref name="Mansuripur2009">{{cite book|author=Masud Mansuripur|title=Classical Optics and Its Applications|date=2009|publisher=Cambridge University Press|isbn=978-0-521-88169-2}}</ref>{{rp|632}}

=== Geology ===
[[File:LvMS-Lvm.jpg|thumb|Photomicrograph of a [[volcanic]] [[sand grain]]; upper picture is plane-polarized light, bottom picture is cross-polarized light, scale box at left-center is 0.25 millimeter.]]
The property of (linear) birefringence is widespread in crystalline [[mineral]]s, and indeed was pivotal in the initial discovery of polarization. In [[mineralogy]], this property is frequently exploited using polarization [[microscope]]s, for the purpose of identifying minerals. See [[optical mineralogy]] for more details.<ref name="Wayne2013">{{cite book|author=Randy O. Wayne|title=Light and Video Microscopy|date=16 December 2013|publisher=Academic Press|isbn=978-0-12-411536-1}}</ref>{{rp|163–164}}

Sound waves in solid materials exhibit polarization. Differential propagation of the three polarizations through the earth is a crucial in the field of [[seismology]]. Horizontally and vertically polarized seismic waves ([[shear waves]]) are termed SH and SV, while waves with longitudinal polarization ([[compressional wave]]s) are termed P-waves.<ref name="Shearer2009">{{cite book|author=Peter M. Shearer|title=Introduction to Seismology|date=2009|publisher=Cambridge University Press|isbn=978-0-521-88210-1}}</ref>{{rp|48–50}}<ref name="SteinWysession2009">{{cite book|author1=Seth Stein|author2=Michael Wysession|title=An Introduction to Seismology, Earthquakes, and Earth Structure|date=1 April 2009|publisher=John Wiley & Sons|isbn=978-1-4443-1131-0}}</ref>{{rp|56–57}}

=== Autopsy ===
Similarly, polarization microscopes can be used to aid in the detection of foreign matter in biological tissue slices if it is birefringent; autopsies often mention (a lack of or presence of) "polarizable foreign debris."<ref>{{Cite journal |last1=Alawi |first1=Faizan |last2=Shields |first2=Bridget E. |last3=Omolehinwa |first3=Temitope |last4=Rosenbach |first4=Misha |date=2020-10-01 |title=Oral Granulomatous Disease |url=https://www.sciencedirect.com/science/article/abs/pii/S0733863520300383 |journal=Dermatologic Clinics |series=Oral Medicine in Dermatology |volume=38 |issue=4 |pages=429–439 |doi=10.1016/j.det.2020.05.004 |pmid=32892852 |issn=0733-8635}}</ref>

=== Chemistry ===
We have seen (above) that the birefringence of a type of crystal is useful in identifying it, and thus detection of linear birefringence is especially useful in [[geology]] and [[mineralogy]]. Linearly polarized light generally has its polarization state altered upon transmission through such a crystal, making it stand out when viewed in between two crossed polarizers, as seen in the photograph, above. Likewise, in chemistry, rotation of polarization axes in a liquid solution can be a useful measurement. In a liquid, linear birefringence is impossible, but there may be circular birefringence when a chiral molecule is in solution. When the right and left handed [[enantiomers]] of such a molecule are present in equal numbers (a so-called [[racemic]] mixture) then their effects cancel out. However, when there is only one (or a preponderance of one), as is more often the case for [[organic molecules]], a net circular birefringence (or ''[[optical activity]]'') is observed, revealing the magnitude of that imbalance (or the concentration of the molecule itself, when it can be assumed that only one enantiomer is present). This is measured using a [[polarimeter]] in which polarized light is passed through a tube of the liquid, at the end of which is another polarizer which is rotated in order to null the transmission of light through it.<ref name="Hecht2002" />{{rp|360–365}}<ref name="VollhardtSchore2003">{{cite book|first1=K. Peter C.|last1=Vollhardt|first2=Neil E.|last2=Schore|author-link2=Neil E. Schore|title=Organic Chemistry: Structure and Function|year=2003|edition=4th|publisher=[[W. H. Freeman]]|isbn=978-0-7167-4374-3|pages=[https://archive.org/details/organicchemistry00voll_0/page/169 169–172]|url=https://archive.org/details/organicchemistry00voll_0/page/169}}</ref>

=== Astronomy ===
{{Main|Polarization in astronomy}}
In many areas of [[astronomy]], the study of polarized electromagnetic radiation from [[outer space]] is of great importance. Although not usually a factor in the [[thermal radiation]] of [[star]]s, polarization is also present in radiation from coherent [[astronomical source]]s (e.g. hydroxyl or methanol [[maser]]s), and incoherent sources such as the large radio lobes in active galaxies, and pulsar radio radiation (which may, it is speculated, sometimes be coherent), and is also imposed upon starlight by scattering from [[interstellar dust]]. Apart from providing information on sources of radiation and scattering, polarization also probes the interstellar magnetic field via [[Faraday rotation]].<ref>{{cite journal|last=Vlemmings|first=W. H. T.|title=A review of maser polarization and magnetic fields|journal=Proceedings of the International Astronomical Union|volume=3|issue=S242|pages=37–46|date=Mar 2007|doi=10.1017/s1743921307012549 |doi-access=free |arxiv = 0705.0885 |bibcode=2007IAUS..242...37V  |bibcode-access=free}}</ref>{{rp|119,124}}<ref name="KarttunenKröger2007">{{cite book|author1=Hannu Karttunen|author2=Pekka Kröger|author3=Heikki Oja|title=Fundamental Astronomy|date=27 June 2007|publisher=Springer|isbn=978-3-540-34143-7}}</ref>{{rp|336–337}} The polarization of the [[cosmic microwave background radiation|cosmic microwave background]] is being used to study the physics of the very early universe.<ref name="boyle">{{cite journal|journal=Physical Review Letters|volume=96|date=2006|pages=111301|first1=Latham A.|last1=Boyle|title=Inflationary predictions for scalar and tensor fluctuations reconsidered|doi=10.1103/PhysRevLett.96.111301|pmid=16605810|last2=Steinhardt|first2=PJ|last3=Turok|first3=N|issue=11|arxiv=astro-ph/0507455 |bibcode=2006PhRvL..96k1301B|s2cid=10424288}}</ref><ref name="tegmark">{{cite journal|first=Max|last=Tegmark|title=What does inflation really predict? |doi=10.1088/1475-7516/2005/04/001|journal=Journal of Cosmology and Astroparticle Physics|volume=0504|issue=4|pages=001|date=2005|arxiv=astro-ph/0410281|bibcode=2005JCAP...04..001T|s2cid=17250080}}</ref> [[Synchrotron radiation]] is inherently polarized. It has been suggested that astronomical sources caused the [[chirality]] of biological molecules on Earth,<ref>{{cite journal |last=Clark |first=S. |date=1999 |title=Polarised starlight and the handedness of Life |journal=American Scientist |volume=97 |issue=4 |pages=336–43 |doi=10.1511/1999.4.336 |bibcode=1999AmSci..87..336C|s2cid=221585816 }}</ref> but chirality selection on inorganic crystals has been proposed as an alternative theory.<ref>{{Cite journal |last1=Hazen |first1=Robert M. |last2=Sholl |first2=David S. |date=2003-06-01 |title=Chiral selection on inorganic crystalline surfaces |url=https://www.nature.com/articles/nmat879 |journal=Nature Materials |language=en |volume=2 |issue=6 |pages=367–374 |doi=10.1038/nmat879 |pmid=12776102 |bibcode=2003NatMa...2..367H |issn=1476-1122}}</ref>