Editing Wave–particle duality (section)

== Observing photons as particles ==
[[File:Photoelectric_effect_in_a_solid_-_diagram.svg|thumb|Photoelectric effect in a solid]]
{{Main|Photoelectric effect|Compton scattering}}
While electrons were thought to be particles until their wave properties were discovered, for photons it was the opposite. In 1887, [[Heinrich Hertz]] observed that when light with sufficient frequency hits a metallic surface, the surface emits [[cathode rays]], what are now called electrons.<ref name="Whittaker1">{{Cite book |last= Whittaker|first=E. T. |title=A History of the Theories of Aether and Electricity: From the Age of Descartes to the Close of the Nineteenth Century |year=1910 |publisher=Longman, Green and Co.}}</ref>{{rp|399}} In 1902, [[Philipp Lenard]] discovered that the maximum possible energy of an ejected electron is unrelated to its [[Intensity (physics)|intensity]].<ref>{{cite journal |last=Wheaton |first=Bruce R. |year=1978 |title=Philipp Lenard and the Photoelectric Effect, 1889-1911 |journal=Historical Studies in the Physical Sciences |volume=9 |pages=299–322 |doi=10.2307/27757381 |jstor=27757381}}</ref> This observation is at odds with classical electromagnetism, which predicts that the electron's energy should be proportional to the intensity of the incident radiation.<ref name="Hawking2">{{cite journal |last1=Hawking |first1=Stephen |url=https://fb2bookfree.com/science/831-the-universe-in-a-nutshell.html |title=The Universe in a Nutshell |date=November 6, 2001 |publisher=Bantam Spectra |others=Impey, C.D. |journal=Physics Today |isbn=978-0553802023 |volume=55 |issue=4 |publication-date=April 2002 |page=80~ |language=en |doi=10.1063/1.1480788 |author-link1=Stephen Hawking |doi-access=free |s2cid-access=free |access-date=December 14, 2020 |orig-date=November 5, 2001 |archive-url=https://web.archive.org/web/20200921192954/https://fb2bookfree.com/science/831-the-universe-in-a-nutshell.html |archive-date=September 21, 2020 |via=Random House Audiobooks |s2cid=120382028}} [[iarchive:StephenHawkingTheUniverseInANutshellBookFi|Alt URL]].</ref>{{rp|24}} In 1905, [[Albert Einstein]] suggested that the energy of the light must occur a finite number of energy quanta.<ref name="Eistein-photoelectric2">{{cite journal |last=Einstein |first=Albert |author-link=Albert Einstein |year=1905 |title=Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt |bibcode-access=free |journal=Annalen der Physik |volume=17 |issue=6 |pages=132–48 |bibcode=1905AnP...322..132E |doi=10.1002/andp.19053220607 |doi-access=free |postscript=, }} translated into English as {{cite book |url-status=dead |chapter-url=http://lorentz.phl.jhu.edu/AnnusMirabilis/AeReserveArticles/eins_lq.pdf |archive-url=https://web.archive.org/web/20090611234106/http://lorentz.phl.jhu.edu/AnnusMirabilis/AeReserveArticles/eins_lq.pdf|archive-date=11 June 2009 |chapter=On a Heuristic Point of View about the Creation and Conversion of Light |title=The Old Quantum Theory |first1=A |last1=Einstein }}  The term "photon" was introduced in 1926.</ref> He postulated that electrons can receive energy from an electromagnetic field only in discrete units (quanta or photons): an amount of [[energy]] ''E'' that was related to the [[frequency]] ''f'' of the light by

: <math>E=hf</math>

[[File:Compton-scattering.svg|right|thumb|200x200px|A photon of wavelength <math>\lambda</math> comes in from the left, collides with a target at rest, and a new photon of wavelength <math>\lambda'</math> emerges at an angle <math>\theta</math>. The target recoils, and the photons have provided momentum to the target.]]
where ''h'' is the [[Planck constant]] (6.626×10<sup>−34</sup> J⋅s). Only photons of a high enough frequency (above a certain ''threshold'' value which, when multiplied by the Planck constant, is the [[work function]]) could knock an electron free. For example, photons of blue light had sufficient energy to free an electron from the metal he used, but photons of red light did not. One photon of light above the threshold frequency could release only one electron; the higher the frequency of a photon, the higher the kinetic energy of the emitted electron, but no amount of light below the threshold frequency could release an electron. Despite confirmation by various experimental observations, the [[photon]] theory (as it came to be called later) remained controversial until [[Arthur Compton]] performed a [[Compton effect|series of experiments]] from 1922 to 1924 demonstrating the momentum of light.<ref name="Whittaker2" />{{rp|211}}

Both discrete (quantized) energies and also momentum are, classically, particle attributes. There are many other examples where photons display particle-type properties, for instance in [[solar sail]]s, where sunlight could propel a space vehicle and [[laser cooling]] where the momentum is used to slow down (cool) atoms. These are a different aspect of wave-particle duality.