Editing Thomas Young (scientist) (section)

===Wave theory of light===
{{see also|Wave–particle duality}}
In Young's own judgment, of his many achievements the most important was to establish the [[wave theory of light]] set out by Christiaan Huygens in his ''[[Treatise on Light]]'' (1690).<ref>{{cite web|title=Thomas Young (1773–1829)|url=http://www.geog.ucsb.edu/~jeff/115a/history/young.html|publisher=UC Santa Barbara|access-date=5 September 2016}}</ref><ref>{{cite web|last=Haidar|first=Riad|title=Thomas Young and the wave theory of light|url=https://www.bibnum.education.fr/sites/default/files/71-young-analysis.pdf|publisher=Bibnum|access-date=5 September 2016|archive-date=15 September 2016|archive-url=https://web.archive.org/web/20160915001601/https://www.bibnum.education.fr/sites/default/files/71-young-analysis.pdf|url-status=dead}}</ref> To do so, he had to overcome the century-old view, expressed in the venerable Newton's ''[[Opticks]]'', that light is a particle. Nevertheless, in the early 19th century Young put forth a number of theoretical reasons supporting the wave theory of light, and he developed two enduring demonstrations to support this viewpoint. With the [[ripple tank]] he demonstrated the idea of [[Interference (wave propagation)|interference]] in the context of water waves. With [[Young's interference experiment]], the predecessor of the [[double-slit experiment]], he demonstrated interference in the context of light as a wave.

[[File:Young-Thomas-Lectures1807-Plate_XXX.jpg|thumb|Plate from "Lectures" of 1802 (RI), pub. 1807]]

Young, speaking on 24 November 1803, to the Royal Society of London, began his now-classic description of the historic experiment:<ref>{{cite book|last=Shamos|first=Morris|title=Great Experiments in Physics|year=1959|publisher=Holt Rinehart and Winston|location=New York|pages=96–101}}</ref>
{{blockquote|The experiments I am about to relate ... may be repeated with great ease, whenever the sun shines, and without any other apparatus than is at hand to every one.<ref>{{cite journal|last=Young|first=Thomas|title=Bakerian Lecture: Experiments and calculations relative to physical optics |journal=Philosophical Transactions of the Royal Society |year=1804 |volume=94 |pages=1–2 |url=https://books.google.com/books?id=7AZGAAAAMAAJ&pg=PA1|bibcode=1804RSPT...94....1Y|doi=10.1098/rstl.1804.0001|s2cid=110408369|doi-access=free}}</ref>}}

In his subsequent paper, titled ''Experiments and Calculations Relative to Physical Optics'' (1804), Young describes an experiment in which he placed a card measuring approximately {{convert|0.85|mm|in}} in a [[beam of light]] from a single opening in a window and observed the fringes of colour in the shadow and to the sides of the card. He observed that placing another card in front or behind the narrow strip so as to prevent the light beam from striking one of its edges caused the fringes to disappear.<ref>{{cite book|last=Magie|first=William Francis|title=A Source Book in Physics|url=https://archive.org/details/in.ernet.dli.2015.449479|publisher=Harvard University Press|year=1935}} p. 309</ref> This supported the contention that light is composed of [[wave]]s.<ref>Both Young and Newton were eventually shown to be partially correct, as neither wave nor particle explanations alone can explain the behaviour of light. See e.g. http://www.single-molecule.nl/notes/light-waves-and-photons/ {{Webarchive|url=https://web.archive.org/web/20160309130103/https://www.single-molecule.nl/notes/light-waves-and-photons/ |date=9 March 2016 }}.</ref>

Young performed and analysed a number of experiments, including interference of light from reflection off nearby pairs of micrometre grooves, from reflection off thin films of soap and oil, and from [[Newton's rings]]. He also performed two important diffraction experiments using fibres and long narrow strips. In his ''Course of Lectures on Natural Philosophy and the Mechanical Arts'' (1807) he gives [[Francesco Maria Grimaldi|Grimaldi]] credit for first observing the fringes in the shadow of an object placed in a beam of light. Within ten years, much of Young's work was reproduced and then extended by [[Augustin-Jean Fresnel]].