Editing Light (section)

=== Wave theory ===
To explain the origin of [[colour]]s, [[Robert Hooke]] (1635–1703) developed a "pulse theory" and compared the spreading of light to that of waves in water in his 1665 work ''[[Micrographia]]'' ("Observation IX"). In 1672 Hooke suggested that light's vibrations could be [[perpendicular]] to the direction of propagation. [[Christiaan Huygens]] (1629–1695) worked out a mathematical wave theory of light in 1678 and published it in his ''[[Treatise on Light]]'' in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the [[luminiferous aether]]. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.<ref>Fokko Jan Dijksterhuis, [https://books.google.com/books?id=cPFevyomPUIC Lenses and Waves: Christiaan Huygens and the Mathematical Science of Optics in the 17th Century], Kluwer Academic Publishers, 2004, {{ISBN|1-4020-2697-8}}</ref> Another supporter of the wave theory was [[Leonhard Euler]]. He argued in ''Nova theoria lucis et colorum'' (1746) that [[diffraction]] could more easily be explained by a wave theory.

[[File:Christiaan Huygens-painting.jpeg|thumb|upright=0.9|[[Christiaan Huygens]]]]
[[File:Young Diffraction.png|thumb|upright=0.9|[[Thomas Young (scientist)|Thomas Young]]'s sketch of a [[double-slit experiment]] showing [[diffraction]]. Young's experiments supported the theory that light consists of waves.]]

The wave theory predicted that light waves could interfere with each other like sound waves (as noted around 1800 by [[Thomas Young (scientist)|Thomas Young]]). Young showed by means of a [[double-slit experiment|diffraction experiment]] that light behaved as waves. He first publicly stated his "general law" of interference in January 1802, in his book ''A Syllabus of a Course of Lectures on Natural and Experimental Philosophy'':<ref>{{Cite journal |last=Mollon |first=J. D. |date=2002 |title=The Origins of the Concept of Interference |jstor=3066507 |journal=Philosophical Transactions: Mathematical, Physical and Engineering Sciences |volume=360 |issue=1794 |pages=807–819 |doi=10.1098/rsta.2001.0968 |pmid=12804280 |bibcode=2002RSPTA.360..807M |issn=1364-503X}}</ref><blockquote>But the general law, by which all these appearances are governed, may be very easily deduced from the interference of two coincident undulations, which either cooperate, or destroy each other, in the same manner as two musical notes produce an alternate intension and remission, in the beating of an imperfect unison.<ref>{{Cite book |last=Young |first=Thomas |title=A Syllabus of a Course of Lectures on Natural and Experimental Philosophy |publisher=W. Savage for The Royal Institution |year=1802 |location=London |publication-date=1802 |page=117}}</ref></blockquote>He also proposed that different colours were caused by different [[wavelength]]s of light and explained colour vision in terms of three-coloured receptors in the eye. 

In 1816 [[André-Marie Ampère]] gave [[Augustin-Jean Fresnel]] an idea that the polarization of light can be explained by the wave theory if light were a [[transverse wave]].<ref>James R. Hofmann, ''André-Marie Ampère: Enlightenment and Electrodynamics'', Cambridge University Press, 1996, p. 222.</ref> Later, Fresnel independently worked out his own wave theory of light and presented it to the [[Académie des Sciences]] in 1817. [[Siméon Denis Poisson]] added to Fresnel's mathematical work to produce a convincing argument in favor of the wave theory, helping to overturn Newton's corpuscular theory.{{dubious|date=June 2018}}<!-- [[Siméon Denis Poisson]] says he was an opponent of the theory --> By the year 1821, Fresnel was able to show via mathematical methods that polarization could be explained by the wave theory of light if and only if light was entirely transverse, with no longitudinal vibration whatsoever.{{Citation needed|date=June 2018}}

The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. The existence of the hypothetical substance luminiferous aether proposed by Huygens in 1678 was cast into strong doubt in the late nineteenth century by the [[Michelson–Morley experiment]].

Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the [[speed of light]] could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was [[Léon Foucault]], in 1850.<ref>{{Cite book |title=Understanding Physics |author1=David Cassidy |author2=Gerald Holton |author3=James Rutherford |publisher=Birkhäuser |year=2002 |isbn=978-0-387-98756-9 |url=https://books.google.com/books?id=rpQo7f9F1xUC&pg=PA382}}</ref> His result supported the wave theory, and the classical particle theory was finally abandoned (only to partly re-emerge in the twentieth century as [[photons]] in [[quantum mechanics|quantum theory]]).