Editing Circular polarization (section)

{{short description|Polarization state}}
[[Image:Circular.Polarization.Circularly.Polarized.Light Left.Hand.Animation.305x190.255Colors.gif|thumb|305px|The [[electric field]] vectors of a traveling circularly polarized electromagnetic wave. This wave is right-handed/clockwise circularly polarized as defined from the point of view of the source, or left-handed/anti-clockwise circularly polarized if defined from the point of view of the receiver.]]

In [[Classical electromagnetism|electrodynamics]], '''circular polarization''' of an [[Electromagnetic radiation|electromagnetic wave]] is a [[Polarization (waves)|polarization]] state in which, at each point, the [[electromagnetic field]] of the wave has a constant magnitude and is rotating at a constant rate in a plane perpendicular to the direction of the wave.

In electrodynamics, the strength and direction of an electric field is defined by its electric field vector. In the case of a circularly polarized wave, the tip of the electric field [[Euclidean vector|vector]], at a given point in space, relates to the phase of the light as it travels through time and space. At any instant of time, the electric field vector of the wave indicates a point on a [[helix]] oriented along the direction of propagation. A circularly polarized wave can rotate in one of two possible senses: ''right-handed circular polarization (RHCP)'' in which the electric field vector rotates in a [[Right-hand rule|right-hand]] sense with respect to the direction of propagation, and ''left-handed circular polarization (LHCP)'' in which the vector rotates in a [[left hand rule|left-hand]] sense.

''Circular polarization'' is a [[limiting case (mathematics)|limiting case]] of ''[[elliptical polarization]]''. The other [[special case]] is the easier-to-understand ''[[linear polarization]]''. All three terms were coined by [[Augustin-Jean Fresnel]], in a memoir read to the [[French Academy of Sciences]] on 9&nbsp;December 1822.<ref name=fresnel-1822z>A. Fresnel, "Mémoire sur la double réfraction que les rayons lumineux éprouvent en traversant les aiguilles de cristal de roche suivant les directions parallèles à l'axe", read 9&nbsp;December 1822; printed in H.&nbsp;de Senarmont, E.&nbsp;Verdet, and L.&nbsp;Fresnel (eds.), ''Oeuvres complètes d'Augustin Fresnel'', vol.&nbsp;1 (1866), pp.{{nnbsp}}731–51; translated as "Memoir on the double refraction that light rays undergo in traversing the needles of quartz in the directions parallel to the axis", {{Zenodo|4745976}}, 2021 (open&nbsp;access); §§9–10.</ref><ref>Académie des Sciences, ''Procès-verbaux des séances de l'Académie tenues depuis la fondation de l'Institut jusqu'au mois d'août 1835'', vol.&nbsp;7 (for 1820–23), Hendaye, Basses Pyrénées: Imprimerie de l'Observatoire d'Abbadia, 1916, p.&nbsp;401.</ref> Fresnel had first described the case of circular polarization, without yet naming it, in 1821.<ref name=fresnel-1821a>A. Fresnel, "Note sur le calcul des teintes que la polarisation développe dans les lames cristallisées" et&nbsp;seq., ''Annales de Chimie et de Physique'', Ser.&nbsp;2, vol.&nbsp;17, pp.&nbsp;102–11 (May&nbsp;1821), 167–96 (June&nbsp;1821), 312–15 ("Postscript", July&nbsp;1821); reprinted (with added section nos.) in H.&nbsp;de Senarmont, E.&nbsp;Verdet, and L.&nbsp;Fresnel (eds.), ''Oeuvres complètes d'Augustin Fresnel'', vol.&nbsp;1 (1866), pp.&nbsp;609–48; translated as "On the calculation of the tints that polarization develops in crystalline plates, &amp;&nbsp;postscript", {{Zenodo|4058004}} (Creative Commons), 2021;&nbsp; author's footnote to §16.</ref>

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.