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.)]]

The term '''''plane of polarization''''' refers to the direction of [[polarization (waves)|polarization]] of ''[[linear polarization|linearly-polarized]]'' light or other [[electromagnetic radiation]]. Unfortunately the term is used with two contradictory meanings. 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>

InFor 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 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.

Fresnel also admitted that, had he not felt constrained by the received terminology, it would have been more natural to define the plane of polarization as the plane containing the vibrations and the direction of propagation.<ref name=fy406>Fresnel, 1822, tr.&nbsp;Young, part&nbsp;7, [https://books.google.com/books?id=N69MAAAAYAAJ&pg=PA406 p.{{nnbsp}}406].</ref> That plane, which became known as the "plane of ''vibration''", is perpendicular to Fresnel's "plane of polarization" but identical with the plane that modern writers tend to call by that name!

It has been argued that the term ''plane of polarization'', because of its historical ambiguity, should be avoided in original writing. One can easily specify the orientation of a particular field vector; and even the term ''plane of vibration'' carries less risk of confusion than ''plane of polarization''.<ref>Born & Wolf, 1970, pp.{{nnbsp}}28,{{hsp}}43.</ref>