Circular polarization: Difference between revisions

Revision as of 17:34, 22 December 2023

Revision as of 17:03, 16 January 2024

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The phenomenon of polarization arises as a consequence of the fact that [[light]] behaves as a two-dimensional [[Transverse wave#Explanation|transverse wave]].

Circular polarization occurs when the two orthogonal electric field component vectors are of equal magnitude and are out of phase by exactly 90°, or one-quarter wavelength.

== Characteristics ==

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Circular polarization is often encountered in the field of optics and, in this section, the electromagnetic wave will be simply referred to as [[light]].

The nature of circular polarization and its relationship to other polarizations is often understood by thinking of the electric field as being divided into two [[Euclidean vector|components]] that are perpendicular to each other. The vertical component and its corresponding plane are illustrated in blue, while the horizontal component and its corresponding plane are illustrated in green. Notice that the rightward (relative to the direction of travel) horizontal component leads the vertical component by one quarter of a [[wavelength]], a 90° phase difference. It is this [[In-phase and quadrature components|quadrature phase]] relationship that creates the [[helix]] and causes the points of maximum magnitude of the vertical component to correspond with the points of zero magnitude of the horizontal component, and vice versa. The result of this alignment are select vectors, corresponding to the helix, which exactly match the maxima of the vertical and horizontal components.

To appreciate how this quadrature [[phase (waves)|phase]] shift corresponds to an electric field that rotates while maintaining a constant magnitude, imagine a dot traveling clockwise in a circle. Consider how the vertical and horizontal [[Displacement (vector)|displacements]] of the dot, relative to the center of the circle, vary [[Sine wave|sinusoidally]] in time and are out of phase by one quarter of a cycle. The displacements are said to be out of phase by one quarter of a cycle because the horizontal maximum displacement (toward the left) is reached one quarter of a cycle before the vertical maximum displacement is reached. Now referring again to the illustration, imagine the center of the circle just described, traveling along the axis from the front to the back. The circling dot will trace out a helix with the displacement toward our viewing left, leading the vertical displacement. Just as the horizontal and vertical displacements of the rotating dot are out of phase by one quarter of a cycle in time, the magnitude of the horizontal and vertical components of the electric field are out of phase by one quarter of a wavelength.

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Just as in the other convention, right-handedness corresponds to a clockwise rotation, and left-handedness corresponds to an anti-clockwise rotation.

Many optics textbooks use this second convention.<ref name="Polarization_in_Spectral_Lines_Section_1.2">Polarization in Spectral Lines. 2004 E. Landi Degl'innocenti, M Landolfi Section 1.2 "When ... the tip of the electric field vector rotates clockwise for an observer facing the radiation source, ... (it will be considered)... positive (or righthanded) circular polarization, Our convention ... agrees with those proposed in the classical textbooks on polarized light by Shurcliff (1952) and by Clarke and Grainger (1971). The same convention is also used, although with some few exceptions, by optical astronomers working in the field of polarimetry. Many radio astronomers, on the other hand, use the opposite convention. [https://books.google.com/books?id=8sl2CkmZNWIC&dq=circular+polarization+conventions&pg=PA5]</ref><ref>HANDBOOK OPTICS Volume I, Devices, Measurements and Properties, Michael Bass Page 272 Footnote: "Right-circularly polarized light is defined as a clockwise rotation of the electric vector when the observer is looking ''against'' the direction the wave is traveling."</ref> It is also used by [[SPIE]]<ref>{{cite web|title=The Polarization Ellipse|url=https://spie.org/publications/fg05_p07-09_polarization_ellipse|website=spie.org|access-date=13 April 2018}}</ref> as well as the [[International Union of Pure and Applied Chemistry]] (IUPAC).<ref>{{cite journal |title=Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006) |author=S. E. Braslavsky |url=https://www.degruyter.com/downloadpdf/journals/pac/79/3/article-p293.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.degruyter.com/downloadpdf/journals/pac/79/3/article-p293.pdf |archive-date=2022-10-09 |url-status=live |journal=Pure and Applied Chemistry |volume=79 |issue=3 |pages=293–465 |date=1 January 2009 |doi=10.1351/pac200779030293|s2cid=96601716 }}</ref>

=== Uses of the two conventions ===

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== FM radio ==

The term "circular polarization" is often used erroneously to describe mixed polarity signals{{Citation needed|date=March 2011}} used mostly in [[FM broadcasting|FM radio]] (87.5 to 108.0 &nbsp;MHz in the USA), in which a vertical and a horizontal component are propagated simultaneously by a single or a combined array. This has the effect of producing greater penetration into buildings and difficult reception areas than a signal with just one plane of polarization. This would be an instance in which the polarization would more appropriately be called random polarization because the polarization at a receiver, although constant, will vary depending on the direction from the transmitter and other factors in the transmitting antenna design. See [[Stokes parameters]].<br />

The term "FM radio" above refers to [[FM broadcasting]], not two-way radio (more properly called [[Land mobile radio system|land mobile radio]]), which uses vertical polarization almost exclusively.

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Weaker sources of circular polarization in nature include multiple scattering by linear polarizers{{dubious|date=June 2021}}, as in the circular polarization of starlight, and selective absorption by [[circular dichroism|circularly dichroic]] media.

Radio emission from stars and pulsars can be strongly circularly polarized{{cncitation needed|date=June 2021}}.

Two species of [[mantis shrimp]] have been reported to be able to detect circular polarized light.<ref>{{cite journal|author1=Tsyr-Huei Chiou |author2=Sonja Kleinlogel |author3=Tom Cronin |author4=Roy Caldwell |author5=Birte Loeffler |author6=Afsheen Siddiqi |author7=Alan Goldizen |author8=Justin Marshall |title=Circular polarization vision in a stomatopod crustacean |journal=[[Current Biology]] |year=2008 |volume=18 |issue=6 |pages=429–34 |doi=10.1016/j.cub.2008.02.066 |pmid=18356053|s2cid=6925705 |doi-access=free }}</ref><ref name="Kleinlogel et al.">{{cite journal |author1=Sonja Kleinlogel |author2=Andrew White |title=The secret world of shrimps: polarisation vision at its best |journal=[[PLoS ONE]] |year=2008 |doi=10.1371/journal.pone.0002190 |volume=3 |issue=5 |pages=e2190 |pmid=18478095 |pmc=2377063 |bibcode=2008PLoSO...3.2190K|arxiv = 0804.2162 |doi-access=free }}</ref>

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*[https://www.youtube.com/watch?v=Q0qrU4nprB0 Comparison of Circular Polarization with Linear and Elliptical Polarizations (YouTube Animation)]

*[http://nagyelte.blogspot.hu/2015/01/reversal-of-handedness-of-circularly.html Reversal of handedness of circularly polarized light by mirror. A demonstration – simple, cheap & instructive]

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