Editing Photoelectric effect (section)

===20th century===
In 1902, Lenard observed that the [[energy]] of individual emitted electrons was independent of the applied light intensity.<ref name="Ref_Lenard" /><ref>{{cite journal |title=Philipp Lenard and the Photoelectric Effect, 1889-1911 |first=Bruce R. |last=Wheaton |journal=Historical Studies in the Physical Sciences |volume=9 |year=1978 |pages=299–322 |doi=10.2307/27757381 |jstor=27757381}}</ref> This appeared to be at odds with Maxwell's [[wave theory of light]], which predicted that the electron energy would be proportional to the [[intensity (physics)|intensity]] of the radiation.

Lenard observed the variation in electron energy with light frequency using a powerful electric arc lamp which enabled him to investigate large changes in intensity. However, Lenard's results were qualitative rather than quantitative because of the difficulty in performing the experiments: the experiments needed to be done on freshly cut metal so that the pure metal was observed, but it oxidized in a matter of minutes even in the partial vacuums he used. The current emitted by the surface was determined by the light's intensity, or brightness: doubling the intensity of the light doubled the number of electrons emitted from the surface.{{citation needed|date=November 2023}}

Initial investigation of the photoelectric effect in gasses by Lenard<ref name="Ref_p">{{cite journal|doi=10.1051/radium:0190800508024001|title=L'ionisation de l'air par la lumière ultra-violette|year=1908|last1=Bloch|first1=E.|journal=Le Radium|volume=5|issue=8|page=240|url=https://hal.archives-ouvertes.fr/jpa-00242302/document}}</ref> were followed up by J. J. Thomson<ref name="Ref_q">{{cite journal|author=Thomson, J. J. |journal=Proc. Camb. Phil. Soc.|volume=14|page=417|year=1907|title=On the Ionisation of Gases by Ultra-Violet Light and on the evidence as to the Structure of Light afforded by its Electrical Effects}}</ref> and then more decisively by Frederic Palmer Jr.<ref name="Ref_r">{{cite journal|doi=10.1038/077582b0|title=Ionisation of Air by Ultra-violet Light|year=1908|last1=Palmer|first1=Frederic|journal=Nature|volume=77|issue=2008|page=582 |bibcode = 1908Natur..77..582P |s2cid=4028617|url=https://zenodo.org/record/1429499|doi-access=free}}</ref><ref>{{cite journal|doi=10.1103/PhysRevSeriesI.32.1|title=Volume Ionization Produced by Light of Extremely Short Wave-Length|year=1911|last1=Palmer|first1=Frederic|journal=Physical Review |series=Series I|volume=32|issue=1|pages=1–22|bibcode = 1911PhRvI..32....1P |url=https://zenodo.org/record/1525061}}</ref> The gas photoemission was studied and showed very different characteristics than those at first attributed to it by Lenard.<ref name="Smithsonian report" />

In 1900, while studying [[black-body radiation]], the German physicist [[Max Planck]] suggested in his "On the Law of Distribution of Energy in the Normal Spectrum"<ref>{{cite journal|last1=Planck|first1=Max|year=1901|title=Ueber das Gesetz der Energieverteilung im Normalspectrum (On the Law of Distribution of Energy in the Normal Spectrum)|journal=Annalen der Physik|volume=4|issue=3|page=553|bibcode=1901AnP...309..553P|doi=10.1002/andp.19013090310|doi-access=free}}</ref> paper that the energy carried by electromagnetic waves could only be released in ''packets'' of energy. In 1905, Albert Einstein published a paper advancing the hypothesis that light energy is carried in discrete quantized packets to explain experimental data from the photoelectric effect. Einstein theorized that the energy in each quantum of light was equal to the frequency of light multiplied by a constant, later called the [[Planck constant]]. A photon above a threshold frequency has the required energy to eject a single electron, creating the observed effect. This was a step in the development of [[quantum mechanics]]. In 1914, [[Robert Andrews Millikan|Robert A. Millikan]]'s highly accurate measurements of the Planck constant from the photoelectric effect supported Einstein's model, even though a corpuscular theory of light was for Millikan, at the time, "quite unthinkable".<ref>{{Cite journal|last=Holton|first=Gerald|date=1999-04-22|title=Centennial Focus: Millikan's Measurement of Planck's Constant|url=https://physics.aps.org/story/v3/st23|journal=Physics|language=en|volume=3|page=23 |doi=10.1103/physrevfocus.3.23|url-access=subscription}}</ref> Einstein was awarded the 1921 [[Nobel Prize in Physics]] for "his discovery of the law of the photoelectric effect",<ref name="Ref_s">{{cite web|title=The Nobel Prize in Physics 1921|url=http://nobelprize.org/nobel_prizes/physics/laureates/1921/index.html|access-date=2008-10-09|publisher=Nobel Foundation}}</ref> and Millikan was awarded the Nobel Prize in 1923 for "his work on the elementary charge of electricity and on the photoelectric effect".<ref>{{cite web|title=The Nobel Prize in Physics 1923|url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1923/press.html|access-date=2015-03-29|publisher=Nobel Foundation}}</ref> In quantum perturbation theory of atoms and solids acted upon by electromagnetic radiation, the photoelectric effect is still commonly analyzed in terms of waves; the two approaches are equivalent because photon or wave absorption can only happen between quantized energy levels whose energy difference is that of the energy of photon.<ref name="Lamb1968">{{cite news|first1 = Willis E. Jr.|last1 = Lamb|author-link = Willis Lamb|last2 = Scully|first2 = Marlan O.|title = The photoelectric effect without photons|location=Coral Gables, FL | institution = Center for Theoretical Physics, University of Miami|quote = we understand the photoeffect as being the result of a classical field falling on a quantized atomic electron |url = https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680009569.pdf|year = 1968}}</ref><ref name="Stefan2003" />

Albert Einstein's mathematical description of how the photoelectric effect was caused by absorption of [[quantum|quanta]] of light was in one of his [[Annus Mirabilis papers]], named "On a Heuristic Viewpoint Concerning the Production and Transformation of Light".<ref>Einstein, A. "[https://sites.pitt.edu/~jdnorton/lectures/Rotman_Summer_School_2013/Einstein_1905_docs/Einstein_Light_Quantum_WikiSource.pdf On a Heuristic Viewpoint Concerning the Emission and Transformation of Light]." Annalen der Physik 17 (1905)</ref> The paper proposed a simple description of ''energy quanta'', and showed how they explained the blackbody radiation spectrum. His explanation in terms of absorption of discrete quanta of light agreed with experimental results. It explained why the energy of photoelectrons was not dependent on incident light ''intensity''. This was a theoretical leap, but the concept was strongly resisted at first because it contradicted the wave theory of light that followed naturally from [[James Clerk Maxwell]]'s equations of electromagnetism, and more generally, the assumption of [[infinite divisibility]] of energy in physical systems.

Einstein's work predicted that the energy of individual ejected electrons increases linearly with the frequency of the light. The precise relationship had not at that time been tested. By 1905 it was known that the energy of photoelectrons increases with increasing ''frequency'' of incident light and is independent of the ''intensity'' of the light. However, the manner of the increase was not experimentally determined until 1914 when Millikan showed that Einstein's prediction was correct.<ref name="Ref_Millikan" />

The photoelectric effect helped to propel the then-emerging concept of wave–particle duality in the nature of light. Light simultaneously possesses the characteristics of both waves and particles, each being manifested according to the circumstances. The effect was impossible to understand in terms of the classical wave description of light,<ref name="Ref_u">Resnick, Robert (1972) ''Basic Concepts in Relativity and Early Quantum Theory'', Wiley, p. 137, {{ISBN|0-471-71702-9}}.</ref><ref name="Ref_">Knight, Randall D. (2004) ''Physics for Scientists and Engineers With Modern Physics: A Strategic Approach'', Pearson-Addison-Wesley, p. 1224, {{ISBN|0-8053-8685-8}}.</ref><ref name="Ref_v">Penrose, Roger (2005) ''The Road to Reality: A Complete Guide to the Laws of the Universe'', Knopf, p. 502, {{ISBN|0-679-45443-8}}</ref> as the energy of the emitted electrons did not depend on the intensity of the incident radiation. Classical theory predicted that the electrons would 'gather up' energy over a period of time, and then be emitted.<ref name="Ref_" /><ref name="Ref_w">Resnick, Robert (1972) ''Basic Concepts in Relativity and Early Quantum Theory'', Wiley, p. 138, {{ISBN|0-471-71702-9}}.</ref>