Editing Christiaan Huygens (section)

==== ''Traité de la Lumière'' ====
[[File:Huyghens - Traité de la lumière - Fig. 12-13.svg|thumb|Refraction of a plane wave, explained using Huygens's principle in ''[[Treatise on Light|Traité de la Lumière]]'' (1690)]]

Huygens is especially remembered in optics for his [[wave]] theory of light, which he first communicated in 1678 to the Académie des sciences in Paris. Originally a preliminary chapter of his ''Dioptrica'', Huygens's theory was published in 1690 under the title ''[[Treatise on Light|Traité de la Lumière]]''<ref>Christiaan Huygens, [https://archive.org/details/bub_gb_X9PKaZlChggC ''Traité de la lumiere''...] (Leiden, Netherlands: Pieter van der Aa, 1690), Chapter 1.</ref> (''Treatise on light''), and contains the first fully mathematized, mechanistic explanation of an unobservable physical phenomenon (i.e., light propagation).<ref name=":6" /><ref name="HuyThomp1912">C. Huygens (1690), translated by Silvanus P. Thompson (1912), ''[[iarchive:treatiseonlight031310mbp|Treatise on Light]]'', London: Macmillan, 1912; [http://www.gutenberg.org/ebooks/14725 Project Gutenberg edition] {{Webarchive|url=https://web.archive.org/web/20200520130041/http://www.gutenberg.org/ebooks/14725|date=20 May 2020}}, 2005; [http://www.grputland.com/2016/06/errata-in-various-editions-of-huygens-treatise-on-light.html Errata] {{Webarchive|url=https://web.archive.org/web/20170610070735/https://archive.org/details/treatiseonlight031310mbp|date=10 June 2017}}, 2016.</ref> Huygens refers to [[Ignace-Gaston Pardies]], whose manuscript on optics helped him on his wave theory.<ref>[https://archive.org/details/bub_gb_X9PKaZlChggC ''Traité de la lumiere''...] (Leiden, Netherlands: Pieter van der Aa, 1690), Chapter 1. From [https://archive.org/details/bub_gb_X9PKaZlChggC/page/n29 page 18]</ref>

The challenge at the time was to explain [[geometrical optics]], as most [[physical optics]] phenomena (such as [[diffraction]]) had not been observed or appreciated as issues. Huygens had experimented in 1672 with double refraction ([[birefringence]]) in the Iceland spar (a [[calcite]]), a phenomenon discovered in 1669 by [[Rasmus Bartholin]]. At first, he could not elucidate what he found but was later able to explain it using his wavefront theory and concept of evolutes.<ref name="HuyThomp1912" /> He also developed ideas on [[caustic (optics)|caustics]].<ref name=":7" /> Huygens assumes that the [[speed of light]] is finite, based on a report by [[Ole Rømer|Ole Christensen Rømer]] in 1677 but which Huygens is presumed to have already believed.<ref name="Smith1987">{{cite book|author=A. Mark Smith|title=Descartes's Theory of Light and Refraction: A Discourse on Method|url=https://books.google.com/books?id=Ei8LAAAAIAAJ&pg=PA70|access-date=11 May 2013|date=1987|publisher=American Philosophical Society|isbn=978-0-87169-773-8|page=70 with note 10|archive-date=16 June 2016|archive-url=https://web.archive.org/web/20160616221725/https://books.google.com/books?id=Ei8LAAAAIAAJ&pg=PA70|url-status=live}}</ref> Huygens's theory posits light as radiating [[wavefront]]s, with the common notion of light rays depicting propagation normal to those wavefronts. Propagation of the wavefronts is then explained as the result of [[spherical wave]]s being emitted at every point along the wave front (known today as the [[Huygens–Fresnel principle]]).<ref>{{Cite journal|last=Shapiro|first=A. E.|date=1973|title=Kinematic Optics: A Study of the Wave Theory of Light in the Seventeenth Century|url=https://www.jstor.org/stable/41133375|journal=Archive for History of Exact Sciences|volume=11|issue=2/3|pages=134–266|doi=10.1007/BF00343533|jstor=41133375|s2cid=119992103|issn=0003-9519}}</ref> It assumed an omnipresent [[luminiferous aether|ether]], with transmission through perfectly elastic particles, a revision of the view of Descartes. The nature of light was therefore a [[longitudinal wave]].<ref name="Smith1987" />

His theory of light was not widely accepted, while Newton's rival [[corpuscular theory of light]], as found in his ''[[Opticks]]'' (1704), gained more support. One strong objection to Huygens's theory was that longitudinal waves have only a single [[polarization (waves)|polarization]] which cannot explain the observed birefringence. However, [[Thomas Young (scientist)|Thomas Young]]'s [[Young's interference experiment|interference experiments]] in 1801, and [[François Arago]]'s detection of the [[Poisson spot]] in 1819, could not be explained through Newton's or any other particle theory, reviving Huygens's ideas and wave models. [[Fresnel]] became aware of Huygens's work and in 1821 was able to explain birefringence as a result of light being not a longitudinal (as had been assumed) but actually a [[transverse wave]].<ref>{{cite book|author1=Darryl J. Leiter|author2=Sharon Leiter|title=A to Z of Physicists|url=https://books.google.com/books?id=Yz1CFkrZ8QMC&pg=PA108|access-date=11 May 2013|date=1 January 2009|publisher=Infobase Publishing|isbn=978-1-4381-0922-0|page=108|archive-date=16 June 2016|archive-url=https://web.archive.org/web/20160616185115/https://books.google.com/books?id=Yz1CFkrZ8QMC&pg=PA108|url-status=live}}</ref> The thus-named Huygens–Fresnel principle was the basis for the advancement of physical optics, explaining all aspects of light propagation until [[James Clerk Maxwell|Maxwell's]] [[History of electromagnetic theory|electromagnetic theory]] culminated in the development of [[quantum mechanics]] and the discovery of the [[photon]].<ref name=":26" /><ref>{{Cite journal|last=Enders|first=P.|date=2009|title=Huygens' Principle as Universal Model of Propagation|url=https://dialnet.unirioja.es/servlet/articulo?codigo=3688899|journal=Latin-American Journal of Physics Education|volume=3|issue=1|pages=4|issn=1870-9095}}</ref>