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Delayed introduction of terms (i) "Poynting vector" and (ii) "plane of vibration" to narrower contexts, because (i) if the propagation direction is taken as the wave-normal direction, that doesn't necessarily coincide with the Poynting vector, and (ii) the planes of polarization and vibration are identical by the new definition but not by the old ones. |
→Fresnel's choice: Added reference to Fresnel's original paper. (Disclosure: the English version is mine, but it's Creative Commons.) |
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[[File:Augustin Fresnel.jpg|thumb|<div style="text-align: center;">Augustin-Jean Fresnel (1788–1827).</div>]]
In 1821, [[Augustin-Jean Fresnel]] announced his hypothesis that light waves are exclusively ''[[transverse wave|transverse]]'' and therefore ''always'' polarized in the sense of having a particular transverse orientation, and that what we call ''unpolarized'' light is in fact light whose orientation is rapidly and randomly changing.<ref>Buchwald, 1989, pp.{{nnbsp}}227–9.</ref><ref name=fresnel-1821a>A. Fresnel, "Note sur le calcul des teintes que la polarisation développe dans les lames cristallisées" et seq., ''Annales de Chimie et de Physique'', Ser. 2, vol. 17, pp. 102–11 (May 1821), 167–96 (June 1821), 312–15 ("Postscript", July 1821); reprinted (with added section nos.) in H. de Sénarmont, E. Verdet, and L. Fresnel (eds.), ''Oeuvres complètes d'Augustin Fresnel'', vol. 1 (1866), pp. 609–48; translated as "On the calculation of the tints that polarization develops in crystalline plates, & postscript", {{Zenodo|4058004}} (Creative Commons), 2021.</ref> Supposing that light waves were analogous to [[s-wave|shear waves]] in [[elasticity (physics)|elastic solids]], and that a higher [[refractive index]] corresponded to a higher [[density]] of the [[luminiferous aether]], he found that he could account for the partial reflection (including polarization by reflection) at the interface between two transparent isotropic media, provided that the vibrations of the aether were perpendicular to the plane of polarization.<ref>Darrigol, 2012, p.{{nnbsp}}212.</ref> Thus the polarization, according to the received definition, was "in" a certain plane if the vibrations were ''perpendicular'' to that plane!
Fresnel himself found this implication inconvenient; later that year he wrote:
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