Editing Polarization (waves) (section)

== Applications and examples ==

=== Polarized sunglasses ===
[[File:Mudflats-polariser.jpg|right|thumb|upright=1.5|Effect of a polarizer on reflection from mud flats. In the picture on the left, the horizontally oriented polarizer preferentially transmits those reflections; rotating the polarizer by 90° (right) as one would view using polarized sunglasses blocks almost all [[specular reflection|specularly reflected]] sunlight.]]
[[File:Test for polarized and non-polarized sunglasses 2.jpg|left|thumb|One can test whether sunglasses are polarized by looking through two pairs, with one perpendicular to the other. If both are polarized, all light will be blocked.]]
Unpolarized light, after being reflected by a specular (shiny) surface, generally obtains a degree of polarization. This phenomenon was observed in the early 1800s by the mathematician [[Étienne-Louis Malus]], after whom [[Malus's law]] is named. Polarizing [[sunglasses]] exploit this effect to reduce glare from reflections by horizontal surfaces, notably the road ahead viewed at a grazing angle.

Wearers of polarized sunglasses will occasionally observe inadvertent polarization effects such as color-dependent birefringent effects, for example in [[toughened glass]] (e.g., car windows) or items made from transparent [[plastic]]s, in conjunction with natural polarization by reflection or scattering. The polarized light from LCD monitors (see below) is extremely conspicuous when these are worn.

=== Sky polarization and photography ===
{{Further|Polarizing filter (Photography)}}
[[File:CircularPolarizer.jpg|right|thumb|upright=1.5|The effects of a [[polarizing filter (photography)|polarizing filter]] (right image) on the sky in a photograph]]

Polarization is observed in the light of the [[Sky#During daytime|sky]], as this is due to sunlight [[light scattering by particles|scattered]] by [[aerosols]] as it passes through [[Earth's atmosphere]]. The [[Rayleigh scattering|scattered]] light produces the brightness and color in clear skies. This partial polarization of scattered light can be used to darken the sky in photographs, increasing the contrast. This effect is most strongly observed at points on the sky making a 90° angle to the Sun. Polarizing filters use these effects to optimize the results of photographing scenes in which reflection or scattering by the sky is involved.<ref name=Hecht2002>{{cite book|last=Hecht|first=Eugene|title=Optics|date=2002| location=United States of America|publisher=Addison Wesley| edition=4th|isbn=0-8053-8566-5}}</ref>{{rp|346–347}}<ref name=Bekefi>{{cite book|last1=Bekefi|first1=George|last2=Barrett|first2=Alan|title=Electromagnetic Vibrations, Waves, and Radiation|url=https://archive.org/details/electromagneticv0000beke|url-access=registration|publisher=MIT Press|location=USA|isbn=0-262-52047-8|date=1977}}</ref>{{rp|495–499}}

[[File:cmglee_London_Embassy_Gardens_pool_polariser.jpg|thumb|upright|Colored fringes in the [[Sky Pool, London|Embassy Gardens Sky Pool]] when viewed through a polarizer, due to stress-induced birefringence in the skylight]]
Sky polarization has been used for orientation in navigation. The [[August Herman Pfund|Pfund sky compass]] was used in the 1950s when navigating near the poles of the [[Earth's magnetic field]] when neither the [[sun]] nor [[star]]s were visible (e.g., under daytime [[cloud]] or [[twilight]]). It has been suggested, controversially, that the [[Viking]]s exploited a similar device (the "[[sunstone (medieval)|sunstone]]") in their extensive expeditions across the [[North Atlantic]] in the 9th–11th centuries, before the arrival of the [[magnetic compass]] from Asia to Europe in the 12th century. Related to the sky compass is the "[[polar clock]]", invented by [[Charles Wheatstone]] in the late 19th century.<ref name="Pye2001">{{cite book|author=J. David Pye|title=Polarised Light in Science and Nature|date=13 February 2001|publisher=CRC Press|isbn=978-0-7503-0673-7}}</ref>{{rp|67–69}}

=== Display technologies ===
The principle of [[liquid-crystal display]] (LCD) technology relies on the rotation of the axis of linear polarization by the liquid crystal array. Light from the [[backlight]] (or the back reflective layer, in devices not including or requiring a backlight) first passes through a linear polarizing sheet. That polarized light passes through the actual liquid crystal layer which may be organized in pixels (for a TV or computer monitor) or in another format such as a [[seven-segment display]] or one with custom symbols for a particular product. The liquid crystal layer is produced with a consistent right (or left) handed chirality, essentially consisting of tiny [[helices]]. This causes circular birefringence, and is engineered so that there is a 90 degree rotation of the linear polarization state. However, when a voltage is applied across a cell, the molecules straighten out, lessening or totally losing the circular birefringence. On the viewing side of the display is another linear polarizing sheet, usually oriented at 90 degrees from the one behind the active layer. Therefore, when the circular birefringence is removed by the application of a sufficient voltage, the polarization of the transmitted light remains at right angles to the front polarizer, and the pixel appears dark. With no voltage, however, the 90 degree rotation of the polarization causes it to exactly match the axis of the front polarizer, allowing the light through. Intermediate voltages create intermediate rotation of the polarization axis and the pixel has an intermediate intensity. Displays based on this principle are widespread, and now are used in the vast majority of televisions, computer monitors and video projectors, rendering the previous [[Cathode-ray tube|CRT]] technology essentially obsolete. The use of polarization in the operation of LCD displays is immediately apparent to someone wearing polarized sunglasses, often making the display unreadable.

In a totally different sense, polarization encoding has become the leading (but not sole) method for delivering separate images to the left and right eye in [[Stereoscopy|stereoscopic]] displays used for [[3D film|3D movies]]. This involves separate images intended for each eye either projected from two different projectors with orthogonally oriented polarizing filters or, more typically, from a single projector with time multiplexed polarization (a fast alternating polarization device for successive frames). [[Polarized 3D glasses]] with suitable polarizing filters ensure that each eye receives only the intended image. Historically such systems used linear polarization encoding because it was inexpensive and offered good separation. However, circular polarization makes separation of the two images insensitive to tilting of the head, and is widely used in 3-D movie exhibition today, such as the system from [[RealD]]. Projecting such images requires screens that maintain the polarization of the projected light when viewed in reflection (such as [[silver screen]]s); a normal diffuse white projection screen causes depolarization of the projected images, making it unsuitable for this application.

Although now obsolete, CRT computer displays suffered from reflection by the glass envelope, causing glare from room lights and consequently poor contrast. Several anti-reflection solutions were employed to ameliorate this problem. One solution utilized the principle of reflection of circularly polarized light. A circular polarizing filter in front of the screen allows for the transmission of (say) only right circularly polarized room light. Now, right circularly polarized light (depending on the [[Circular polarization#From the point of view of the source|convention]] used) has its electric (and magnetic) field direction rotating clockwise while propagating in the +z direction. Upon reflection, the field still has the same direction of rotation, but now propagation is in the −z direction making the reflected wave ''left'' circularly polarized. With the right circular polarization filter placed in front of the reflecting glass, the unwanted light reflected from the glass will thus be in very polarization state that is ''blocked'' by that filter, eliminating the reflection problem. The reversal of circular polarization on reflection and elimination of reflections in this manner can be easily observed by looking in a mirror while wearing 3-D movie glasses which employ left- and right-handed circular polarization in the two lenses. Closing one eye, the other eye will see a reflection in which it cannot see itself; that lens appears black. However, the other lens (of the closed eye) will have the correct circular polarization allowing the closed eye to be easily seen by the open one.

=== Radio transmission and reception{{anchor|Antennas}} ===
{{See also|Antenna (radio)#Polarization}}

All [[radio]] (and microwave) [[antenna (radio)|antennas]] used for transmitting or receiving are intrinsically polarized. They transmit in (or receive signals from) a particular polarization, being totally insensitive to the opposite polarization; in certain cases that polarization is a function of direction. Most antennas are nominally linearly polarized, but elliptical and circular polarization is a possibility. In the case of linear polarization, the same kind of filtering as described above, is possible. In the case of elliptical polarization (circular polarization is in reality just a kind of elliptical polarization where the length of both elasticity factors is the same), filtering out a single angle (e.g. 90°) will have virtually no impact as the wave at any time can be in any of the 360 degrees.

The vast majority of antennas are linearly polarized. In fact it can be shown from considerations of symmetry that an antenna that lies entirely in a plane which also includes the observer, can ''only'' have its polarization in the direction of that plane. This applies to many cases, allowing one to easily infer such an antenna's polarization at an intended direction of propagation. So a typical rooftop [[Yagi-Uda antenna|Yagi]] or [[log-periodic antenna]] with horizontal conductors, as viewed from a second station toward the horizon, is necessarily horizontally polarized. But a vertical "[[whip antenna]]" or AM broadcast tower used as an antenna element (again, for observers horizontally displaced from it) will transmit in the vertical polarization. A [[turnstile antenna]] with its four arms in the horizontal plane, likewise transmits horizontally polarized radiation toward the horizon. However, when that same turnstile antenna is used in the "axial mode" (upwards, for the same horizontally-oriented structure) its radiation is circularly polarized. At intermediate elevations it is elliptically polarized.

Polarization is important in radio communications because, for instance, if one attempts to use a horizontally polarized antenna to receive a vertically polarized transmission, the signal strength will be substantially reduced (or under very controlled conditions, reduced to nothing). This principle is used in [[satellite television]] in order to double the channel capacity over a fixed frequency band. The same frequency channel can be used for two signals broadcast in opposite polarizations. By adjusting the receiving antenna for one or the other polarization, either signal can be selected without interference from the other.

Especially due to the presence of the [[Surface wave#Ground wave|ground]], there are some differences in propagation (and also in reflections responsible for TV [[Ghosting (television)|ghosting]]) between horizontal and vertical polarizations. AM and FM broadcast radio usually use vertical polarization, while television uses horizontal polarization. At low frequencies especially, horizontal polarization is avoided. That is because the phase of a horizontally polarized wave is reversed upon reflection by the ground. A distant station in the horizontal direction will receive both the direct and reflected wave, which thus tend to cancel each other. This problem is avoided with vertical polarization. Polarization is also important in the transmission of [[radar]] pulses and reception of radar reflections by the same or a different antenna. For instance, back scattering of radar pulses by rain drops can be avoided by using circular polarization. Just as specular reflection of circularly polarized light reverses the handedness of the polarization, as discussed above, the same principle applies to scattering by objects much smaller than a wavelength such as rain drops. On the other hand, reflection of that wave by an irregular metal object (such as an airplane) will typically introduce a change in polarization and (partial) reception of the return wave by the same antenna.

The effect of [[plasma (physics)|free electrons]] in the [[ionosphere]], in conjunction with the [[earth's magnetic field]], causes [[Faraday rotation#Faraday rotation in the ionosphere|Faraday rotation]], a sort of circular birefringence. This is the same mechanism which can rotate the axis of linear polarization by electrons in [[interstellar space]] as mentioned [[#Astronomy|below]]. The magnitude of Faraday rotation caused by such a plasma is greatly exaggerated at lower frequencies, so at the higher microwave frequencies used by satellites the effect is minimal. However, medium or [[short wave]] transmissions received following [[sky wave|refraction by the ionosphere]] are strongly affected. Since a wave's path through the ionosphere and the earth's magnetic field vector along such a path are rather unpredictable, a wave transmitted with vertical (or horizontal) polarization will generally have a resulting polarization in an arbitrary orientation at the receiver.
[[File:Oceano Atlantico 1989.jpg|thumb|upright|Circular polarization through an airplane plastic window, 1989]]

=== Polarization and vision ===
Many [[animal]]s are capable of perceiving some of the components of the polarization of light, e.g., linear horizontally polarized light. This is generally used for navigational purposes, since the linear polarization of sky light is always perpendicular to the direction of the sun. This ability is very common among the [[insect]]s, including [[bee]]s, which use this information to orient their [[Bee learning and communication|communicative dances]].<ref name="Pye2001" />{{rp|102–103}} Polarization sensitivity has also been observed in species of [[octopus]], [[squid]], [[cuttlefish]], and [[mantis shrimp]].<ref name="Pye2001" />{{rp|111–112}} In the latter case, one species measures all six orthogonal components of polarization, and is believed to have optimal polarization vision.<ref>{{cite journal|author=Sonja Kleinlogel|author2=Andrew White| title=The secret world of shrimps: polarisation vision at its best| journal=PLOS ONE| date=2008|pmid=18478095|pmc=2377063| doi=10.1371/journal.pone.0002190 |doi-access=free|volume=3|issue=5|pages=e2190|bibcode=2008PLoSO...3.2190K |arxiv=0804.2162}}</ref> The rapidly changing, vividly colored skin patterns of cuttlefish, used for communication, also incorporate polarization patterns, and mantis shrimp are known to have polarization selective reflective tissue. Sky polarization was thought to be perceived by [[pigeon]]s, which was assumed to be one of their aids in [[homing pigeon|homing]], but research indicates this is a popular myth.<ref>{{Cite journal|last1=Nuboer|first1=J. F. W.|last2=Coemans|first2=M. a. J. M.|last3=Vos Hzn|first3=J. J.|date=1995-02-01|title=No evidence for polarization sensitivity in the pigeon electroretinogram|url=https://jeb.biologists.org/content/198/2/325|journal=Journal of Experimental Biology|language=en|volume=198|issue=2|pages=325–335|doi=10.1242/jeb.198.2.325|issn=0022-0949|pmid=9317897|doi-access=free|bibcode=1995JExpB.198..325H |access-date=2019-08-27|archive-date=2019-08-27|archive-url=https://web.archive.org/web/20190827032524/https://jeb.biologists.org/content/198/2/325|url-status=live}}</ref>

The naked [[human eye]] is weakly sensitive to polarization, without the need for intervening filters. Polarized light creates a very faint pattern near the center of the visual field, called [[Haidinger's brush]]. This pattern is very difficult to see, but with practice one can learn to detect polarized light with the naked eye.<ref name="Pye2001" />{{rp|118}}

=== Angular momentum using circular polarization ===
It is well known that electromagnetic radiation carries a certain linear [[momentum]] in the direction of propagation. In addition, however, light carries a certain [[angular momentum]] if it is circularly polarized (or partially so). In comparison with lower frequencies such as microwaves, the amount of [[Spin angular momentum of light|angular momentum in light]], even of pure circular polarization, compared to the same wave's linear momentum (or [[radiation pressure]]) is very small and difficult to even measure. However, it was utilized in an experiment to achieve speeds of up to 600 million revolutions per minute.<ref>{{cite web | title='Fastest spinning object' created | website=BBC News | date=2013-08-28 | url=https://www.bbc.com/news/uk-scotland-edinburgh-east-fife-23861397 | access-date=2019-08-27 | archive-date=2019-09-10 | archive-url=https://web.archive.org/web/20190910093348/https://www.bbc.com/news/uk-scotland-edinburgh-east-fife-23861397 | url-status=live }}</ref><ref>{{Cite journal|title=Laser-induced rotation and cooling of a trapped microgyroscope in vacuum|journal = Nature Communications|volume = 4|pages = 2374|first1=Kishan|last1=Dholakia|first2=Michael|last2=Mazilu|first3=Yoshihiko|last3=Arita|date=August 28, 2013|doi=10.1038/ncomms3374|pmid = 23982323|pmc = 3763500|hdl = 10023/4019|bibcode = 2013NatCo...4.2374A}}</ref>