Plane of polarization: Difference between revisions

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[[File:Field-vectors-and-propagation-directions.svg|thumb|300px|'''Fig.{{nnbsp}}1''':{{big| }}Field vectors ('''E''',{{hsp}}'''D''',{{hsp}}'''B''',{{hsp}}'''H''') and propagation directions (ray and wave-normal) for linearly-polarized plane electromagnetic waves in a non-magnetic birefringent crystal.{{r|lunney-weaire-2006}} The plane of vibration, containing both electric vectors ('''E''' & '''D''') and both propagation vectors, is sometimes called the "plane of polarization" by modern authors. Fresnel's "plane of polarization", traditionally used in optics, is the plane containing the magnetic vectors ('''B''' & '''H''') and the ''wave-normal''. Malus's original "plane of polarization" was the plane containing the magnetic vectors and the ''ray''.  (In an isotropic medium,  {{math|''θ'' {{=}} 0}}  and Malus's plane merges with Fresnel's.)]]

For [[light]] and other [[electromagnetic radiation]], the '''plane of polarization''' is the [[plane (geometry)|plane]] spanned by the direction of propagation and either the [[electric vector]] or the [[magnetic vector]], depending on the convention. It can be defined for [[polarization (physics)|polarized]] light, remains fixed in space for ''[[linear polarization|linearly-polarized]]'' light, and undergoes [[axial rotation]] for ''[[circular polarization|circularly-polarized]]'' light.

Unfortunately the two [[plane (geometry)|plane]] conventions are contradictory. As originally defined by [[Étienne-Louis Malus]] in 1811,<ref name=buch54>Buchwald, 1989, p.{{hsp}}54.</ref> the plane of polarization coincided (although this was not known at the time) with the plane containing the direction of propagation and the ''magnetic'' vector.<ref>Stratton, 1941, p.{{hsp}}280; Born & Wolf, 1970, pp.{{nnbsp}}43,{{tsp}}681.</ref> In modern literature, the term ''plane of polarization'', if it is used at all, is likely to mean the plane containing the direction of propagation and the ''electric'' vector,<ref name=luntz/> because the electric field has the greater propensity to interact with matter.<ref name="bw28">Born & Wolf, 1970, p.{{hsp}}28.</ref>

For waves in a [[birefringence|birefringent]] (doubly-refractive) crystal, under the old definition, one must also specify whether the direction of propagation means the ray direction ([[Poynting vector]]) or the wave-[[normal (geometry)|normal]] direction, because these directions generally differ and are both perpendicular to the magnetic vector (Fig.{{nnbsp}}1). Malus, as an adherent of the [[corpuscular theory of light]], could only choose the ray direction. But [[Augustin-Jean Fresnel]], in his successful effort to explain double refraction under the [[wave theory of light|wave theory]] (1822 onward), found it more useful to choose the wave-normal direction, with the result that the supposed vibrations of the medium were then consistently perpendicular to the plane of polarization.<ref name=fh318>Fresnel, 1827, tr.&nbsp;Hobson, p.{{nnbsp}}318.</ref> In an [[isotropy|isotropic]] medium such as air, the ray and wave-normal directions are the same, and Fresnel's modification makes no difference.