Editing Photon (section)

== Nomenclature ==
[[File:Photoelectric_effect_in_a_solid_-_diagram.svg|alt=|thumb|[[Photoelectric effect]]: the emission of electrons from a metal plate caused by light quanta – photons]]
The word [[Quantum|''quanta'']] (singular ''quantum,'' Latin for ''[[wikt:quantum|how much]]'') was used before 1900 to mean particles or amounts of different [[Quantity|quantities]], including [[electron|electricity]]. In 1900, the German physicist [[Max Planck]] was studying [[black-body radiation]], and he suggested that the experimental observations, specifically at [[ultraviolet catastrophe|shorter wavelengths]], would be explained if the energy was "made up of a completely determinate number of finite equal parts", which he called "energy elements".<ref>{{cite journal|last=Kragh |first=Helge |author-link=Helge Kragh |title=Max Planck: the reluctant revolutionary |journal=[[Physics World]] |date=2000-12-01 |volume=13 |number=12 |pages=31–36 |doi=10.1088/2058-7058/13/12/34}}</ref> In 1905, [[Albert Einstein]] published a paper in which he proposed that many light-related phenomena—including black-body radiation and the [[photoelectric effect]]—would be better explained by modelling electromagnetic waves as consisting of spatially localized, discrete energy quanta.<ref name="Einstein1905">{{Cite journal |last=Einstein |first=Albert |author-link=Albert Einstein |year=1905 |title=Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt |url=http://www.physik.uni-augsburg.de/annalen/history/einstein-papers/1905_17_132-148.pdf |url-status=live |journal=[[Annalen der Physik]] |language=de |volume=17 |issue=6 |pages=132–148 |bibcode=1905AnP...322..132E |doi=10.1002/andp.19053220607 |archive-url=https://web.archive.org/web/20150924072915/http://www.physik.uni-augsburg.de/annalen/history/einstein-papers/1905_17_132-148.pdf |archive-date=2015-09-24 |access-date=2010-08-25 |quote=According to this picture, the energy of a light wave emitted from a point source is not spread continuously over ever larger volumes, but consists of a finite number of energy quanta that are spatially localized at points of space, move without dividing and are absorbed or generated only as a whole. |doi-access=free}} An [[wikisource:Translation:On a Heuristic Point of View about the Creation and Conversion of Light|English translation]] is available from [[Wikisource]].</ref> He called these ''a light quantum'' (German: ''ein Lichtquant'').<ref>{{cite book |first=Max |last=Planck |title=The Origin and Development of the Quantum Theory |year=1922 |publisher=Clarendon Press |section=via Google Books |section-url={{google books |plainurl=y |id=4UC4AAAAIAAJ}} |url=https://archive.org/details/origindevelopmen00planrich |via=Internet Archive (archive.org, 2007-03-01)}}</ref>

The name ''photon'' derives from the [[Greek language|Greek word]] for light, ''{{lang|grc|φῶς}}'' (transliterated ''phôs''). The name was used 1916 by the American physicist and psychologist [[Leonard T. Troland]] for a unit of illumination of the [[retina]] and in several other contexts before being adopted for physics.<ref name="kragh"/> The use of the term ''photon'' for the light quantum was popularized by [[Gilbert N. Lewis]], who used the term in a letter to ''[[Nature (journal)|Nature]]'' on 18&nbsp;December 1926.<ref>{{cite journal |last=Lewis |first=Gilbert N. |author-link=Gilbert N. Lewis |date=18 December 1926 |title=The conservation of photons |journal=Nature |volume=118 |issue=2981 |pages=874–875 |bibcode=1926Natur.118..874L |doi=10.1038/118874a0 |eissn=1476-4687 |s2cid=4110026}}</ref> Arthur Compton, who had performed a key experiment demonstrating light quanta, cited Lewis in the 1927 [[Solvay conference]] proceedings for suggesting the name ''photon''. Einstein never did use the term.<ref name="kragh"/>  

In physics, a photon is usually denoted by the symbol [[gamma|{{math|γ}}]] (the [[Greek alphabet|Greek letter]] [[gamma]]). This symbol for the photon probably derives from [[gamma ray]]s, which were discovered in 1900 by [[Paul Ulrich Villard|Paul Villard]],<ref>{{cite journal |last=Villard |first=Paul Ulrich |author-link=Paul Ulrich Villard |year=1900 |title=Sur la réflexion et la réfraction des rayons cathodiques et des rayons déviables du radium |journal=[[Comptes Rendus des Séances de l'Académie des Sciences]] |language=fr |volume=130 |pages=1010–1012}}</ref><ref>{{cite journal |last=Villard |first=Paul Ulrich |author-link=Paul Ulrich Villard |year=1900 |title=Sur le rayonnement du radium |journal=[[Comptes Rendus des Séances de l'Académie des Sciences]] |language=fr |volume=130 |pages=1178–1179}}</ref> named by [[Ernest Rutherford]] in 1903, and shown to be a form of [[electromagnetic radiation]] in 1914 by Rutherford and [[Edward Andrade]].<ref>{{cite journal |last1=Rutherford |first1=Ernest |author-link=Ernest Rutherford |last2=Andrade |first2=Edward N.C. |author-link2=Edward Andrade |year=1914 |title=The wavelength of the soft gamma rays from Radium B |url=https://zenodo.org/record/2278669 |journal=[[Philosophical Magazine]] |volume=27 |issue=161 |pages=854–868 |doi=10.1080/14786440508635156 |access-date=2019-08-25 |archive-date=2020-03-08 |archive-url=https://web.archive.org/web/20200308183926/https://zenodo.org/record/2278669 |url-status=live }}</ref> In [[chemistry]] and [[optical engineering]], photons are usually symbolized by {{mvar|hν}}, which is the [[photon energy]], where {{mvar|h}} is the [[Planck constant]] and the [[Greek alphabet|Greek letter]] {{mvar|ν}} ([[Nu (letter)|nu]]) is the photon's [[frequency]].<ref name="Liddle2015">{{cite book |author=Liddle |first=Andrew |url=https://books.google.com/books?id=6n64CAAAQBAJ&pg=PA16 |title=An Introduction to Modern Cosmology |date=2015 |publisher=John Wiley & Sons |isbn=978-1-118-69025-3 |page=16 |language=en |access-date=2017-02-27 |archive-date=2024-05-13 |archive-url=https://web.archive.org/web/20240513012456/https://books.google.com/books?id=6n64CAAAQBAJ&pg=PA16#v=onepage&q&f=false |url-status=live }}</ref>