Editing Circular polarization (section)

== Handedness conventions {{Anchor|Left/Right}} ==

[[Image:Circular.Polarization.Circularly.Polarized.Light Left.Hand.Animation.305x190.255Colors.gif|thumb|305px|A right-handed/clockwise circularly polarized wave as defined from the point of view of the source. It would be considered left-handed/anti-clockwise circularly polarized if defined from the point of view of the receiver.]]
 [[Image:Circular.Polarization.Circularly.Polarized.Light Right.Handed.Animation.305x190.255Colors.gif|thumb|305px|A left-handed/anti-clockwise circularly polarized wave as defined from the point of view of the source. It would be considered right-handed/clockwise circularly polarized if defined from the point of view of the receiver.]]

Circular polarization may be referred to as right-handed or left-handed, and clockwise or anti-clockwise, depending on the direction in which the electric field vector rotates. Unfortunately, two opposing historical conventions exist.

=== From the point of view of the source ===

Using this convention, polarization is defined from the point of view of the source. When using this convention, left- or right-handedness is determined by pointing one's left or right thumb {{em|away}} from the source, in the {{em|same}} direction that the wave is propagating, and matching the curling of one's fingers to the direction of the temporal rotation of the field at a given point in space. When determining if the wave is clockwise or anti-clockwise circularly polarized, one again takes the point of view of the source, and while looking {{em|away}} from the source and in the {{em|same}} direction of the wave's propagation, one observes the direction of the field's temporal rotation.

Using this convention, the electric field vector of a left-handed circularly polarized wave is as follows:
<math> \left( E_x ,\, E_y ,\, E_z \right) \propto \left(\cos \frac{2 \pi}{\lambda} \left(c t - z \right),\, -\sin \frac{2 \pi}{\lambda} \left(c t - z \right),\, 0 \right) . </math>

As a specific example, refer to the circularly polarized wave in the first animation. Using this convention, that wave is defined as right-handed because when one points one's right thumb in the same direction of the wave's propagation, the fingers of that hand curl in the same direction of the field's temporal rotation. It is considered clockwise circularly polarized because, from the point of view of the source, looking in the same direction of the wave's propagation, the field rotates in the clockwise direction. The second animation is that of left-handed or anti-clockwise light, using this same convention.

This convention is in conformity with the [[Institute of Electrical and Electronics Engineers]] (IEEE) standard and, as a result, it is generally used in the engineering community.<ref>IEEE Std 149-1979 (R2008), "IEEE Standard Test Procedures for Antennas". Reaffirmed December 10, 2008, Approved December 15, 1977, IEEE-SA Standards Board. Approved October 9, 2003, American National Standards Institute. {{ISBN|0-471-08032-2}}. {{doi|10.1109/IEEESTD.1979.120310}}, sec. 11.1, p.&nbsp;61."the sense of polarization, or handedness ... is called right handed (left handed) if the direction of rotation is clockwise (anti-clockwise) for an observer looking in the direction of propagation"</ref><ref name=Orfanidis>Electromagnetic Waves & Antennas – S. J. Orfanidis: Footnote p.45, "most engineering texts use the IEEE convention and most physics texts, the opposite convention."</ref><ref>Electromagnetic Waves & Antennas – S. J. Orfanidis Pg 44 "Curl the fingers of your left and right hands into a fist and point both thumbs ''towards'' the direction of propagation"</ref>

Quantum physicists also use this convention of handedness because it is consistent with their convention of handedness for a particle's spin.<ref name="Lectures_on_Physics(Vol_1_ch_33-1)" >Lectures on Physics Feynman (Vol. 1, ch.33-1) "If the end of the electric vector, when we look at it as the light comes straight toward us, goes around in an anti-clockwise direction, we call it right-hand circular polarization. ... Our convention for labeling left-hand and right-hand circular polarization is consistent with that which is used today for all the other particles in physics which exhibit polarization (e.g., electrons). However, in some books on optics the opposite conventions are used, so one must be careful."</ref>

Radio astronomers also use this convention in accordance with an [[International Astronomical Union|International Astronomical Union (IAU)]] resolution made in 1973.<ref name=IAU>IAU General Assembly Meeting, 1973, Commission 40 (Radio Astronomy/Radioastronomie), 8. POLARIZATION DEFINITIONS -- "A working Group chaired by Westerhout was convened to discuss the definition of polarization brightness temperatures used in the description of polarized extended objects and the galactic
background. The following resolution was adopted by Commissions 25 and 40: 'RESOLVED, that the frame of reference for the Stokes parameters is that of Right Ascension and Declination with the position angle of electric-vector maximum, q, starting from North and increasing through East. Elliptical polarization is defined in conformity with the definitions of the Institute of Electrical and Electronics Engineers (IEEE Standard 211, 1969). This means that the polarization of incoming radiation, for which the position angle, q, of the electric vector, measured at a fixed point in space, increases with time, is described as right-handed and positive.'"</ref>

=== From the point of view of the receiver ===

In this alternative convention, polarization is defined from the point of view of the receiver. Using this convention, left- or right-handedness is determined by pointing one's left or right thumb {{em|toward}} the source, {{em|against}} the direction of propagation, and then matching the curling of one's fingers to the temporal rotation of the field.

When using this convention, in contrast to the other convention, the defined handedness of the wave matches the handedness of the screw type nature of the field in space. Specifically, if one freezes a right-handed wave in time, when one curls the fingers of one's right hand around the helix, the thumb will point in the direction of progression for the helix, given the sense of rotation. Note that, in the context of the nature of all screws and helices, it does not matter in which direction you point your thumb when determining its handedness.

When determining if the wave is clockwise or anti-clockwise circularly polarized, one again takes the point of view of the receiver and, while looking {{em|toward}} the source, {{em|against}} the direction of propagation, one observes the direction of the field's temporal rotation.

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 ===

As stated earlier, there is significant confusion with regards to these two conventions. As a general rule, the engineering, quantum physics, and radio astronomy communities use the first convention, in which the wave is observed from the point of view of the source.<ref name=Orfanidis /><ref name="Lectures_on_Physics(Vol_1_ch_33-1)" /><ref name=IAU /> In many physics textbooks dealing with optics, the second convention is used, in which the light is observed from the point of view of the receiver.<ref name="Lectures_on_Physics(Vol_1_ch_33-1)" /><ref name="Polarization_in_Spectral_Lines_Section_1.2" />

To avoid confusion, it is good practice to specify "as defined from the point of view of the source" or "as defined from the point of view of the receiver" when discussing polarization matters.

The archive of the [https://web.archive.org/web/20090822015912/http://www.its.bldrdoc.gov/fs-1037/ US Federal Standard 1037C] proposes two contradictory conventions of handedness.<ref>In one location it is stated..."Note 1. ... In general, the figure, i.e., polarization, is elliptical and is traced in a clockwise or anti-clockwise sense, as viewed in the direction of propagation. ... Rotation of the electric vector in a clockwise sense is designated right-hand polarization, and rotation in an anti-clockwise sense is designated left-hand polarization. "[http://www.its.bldrdoc.gov/fs-1037//dir-028/_4059.htm] {{Webarchive|url=https://web.archive.org/web/20110514080812/http://www.its.bldrdoc.gov/fs-1037/dir-028/_4059.htm|date=2011-05-14}} In another location it is stated... "Note 4: Circular polarization may be referred to as "right-hand" or "left-hand", depending on whether the helix describes the thread of a right-hand or left-hand screw, respectively". [http://www.its.bldrdoc.gov/fs-1037/dir-007/_0972.htm] {{Webarchive|url=https://web.archive.org/web/20110606113103/http://www.its.bldrdoc.gov/fs-1037/dir-007/_0972.htm|date=2011-06-06}}</ref>

Note that the IEEE defines RHCP and LHCP the opposite as those used by physicists.  The IEEE 1979 Antenna Standard will show RHCP on the South Pole of the Poincare Sphere. The IEEE defines RHCP using the right hand with thumb pointing in the direction of transmit, and the fingers showing the direction of rotation of the E field with time.  The rationale for the opposite conventions used by Physicists and Engineers is that Astronomical Observations are always done with the incoming wave traveling toward the observer, where as for most engineers, they are assumed to be standing behind the transmitter watching the wave traveling away from them.  This article is not using the IEEE 1979 Antenna Standard and is not using the +t convention typically used in IEEE work.