Editing Photoelectric effect (section)

===19th century===
In 1839, [[A. E. Becquerel|Alexandre Edmond Becquerel]] discovered the related [[photovoltaic effect]] while studying the effect of light on [[electrolytic cell]]s.<ref name="Petrova-KochHezel2009">{{cite book|author1=Vesselinka Petrova-Koch|author2=Rudolf Hezel|author3=Adolf Goetzberger|chapter=Milestones of Solar Conversion and Photovoltaics |series=Springer Series in Optical Sciences |volume=140 |title=High-Efficient Low-Cost Photovoltaics: Recent Developments|year=2009|publisher=Springer|isbn=978-3-540-79358-8|pages=1–|doi=10.1007/978-3-540-79359-5_1|s2cid=108793685 }}</ref> Though not equivalent to the photoelectric effect, his work on [[photovoltaics]] was instrumental in showing a strong relationship between light and electronic properties of materials. In 1873, [[Willoughby Smith]] discovered [[photoconductivity]] in [[selenium]] while testing the metal for its high resistance properties in conjunction with his work involving submarine telegraph cables.<ref name="Smith1873">{{cite journal|author=Smith, W.|url=https://www.histv.net/willoughby-smith|year=1873|title=Effect of Light on Selenium during the passage of an Electric Current|doi=10.1038/007303e0|journal=Nature|page=303|volume=7|issue=173|bibcode = 1873Natur...7R.303. |doi-access=free}}</ref>

[[Julius Elster|Johann Elster]] (1854–1920) and [[Hans Friedrich Geitel|Hans Geitel]] (1855–1923), students in [[Heidelberg]], investigated the effects produced by light on electrified bodies and developed the first practical photoelectric cells that could be used to measure the intensity of light.<ref name="Ref_e">Asimov, A. (1964) ''[[Asimov's Biographical Encyclopedia of Science and Technology]]'', Doubleday, {{ISBN|0-385-04693-6}}.</ref><ref name="BudWarner1998">{{cite book|author1=Robert Bud|author2=Deborah Jean Warner|title=Instruments of Science: An Historical Encyclopedia|year=1998|publisher=Science Museum, London, and National Museum of American History, Smithsonian Institution|isbn=978-0-8153-1561-2}}</ref>{{rp|458}} They arranged metals with respect to their power of discharging negative electricity: [[rubidium]], [[potassium]], [[alloy]] of potassium and sodium, [[sodium]], [[lithium]], [[magnesium]], [[thallium]] and [[zinc]]; for [[copper]], [[platinum]], [[lead]], [[iron]], [[cadmium]], [[carbon]], and [[Mercury (element)|mercury]] the effects with ordinary light were too small to be measurable. The order of the metals for this effect was the same as in Volta's series for contact-electricity, the most electropositive metals giving the largest photo-electric effect.
[[File:Gold leaf electroscope 1869.png|alt=|thumb|282x282px|Gold leaf [[electroscope]] demonstrating the photoelectric effect. When the electroscope disk is negatively charged with excess electrons, the gold leaves mutually repel. If high-energy light (such as ultraviolet) is then shone on the disk, electrons are emitted by the photoelectric effect and the leaf repulsion ceases. But if the light used has insufficient energy to stimulate electron emission, the leaves stay separated regardless of duration.]]
In 1887, [[Heinrich Hertz]] observed the photoelectric effect<ref name="Ref_f">
{{cite journal|last=Hertz|first=Heinrich|date=1887|title=Ueber einen Einfluss des ultravioletten Lichtes auf die electrische Entladung|url=https://zenodo.org/record/1423827|journal=[[Annalen der Physik]]|volume=267|issue=8|pages=983–1000|bibcode=1887AnP...267..983H|doi=10.1002/andp.18872670827}}
</ref> and reported on the production and reception<ref>{{Cite journal|last=Hertz|first=H.|date=1887|title=Ueber sehr schnelle electrische Schwingungen|journal=Annalen der Physik und Chemie|language=en|volume=267|issue=7|pages=421–448|doi=10.1002/andp.18872670707|bibcode=1887AnP...267..421H|issn=0003-3804|url=https://zenodo.org/record/1423823}}</ref> of electromagnetic waves.<ref name="Smithsonian report">{{cite book |last1=Bloch |first1=Eugene |title=Annual Report Of The Board Of Regents Of The Smithsonian Institution 1913 |date=1914 |publisher=Smithsonian Institution |location=Washington, DC |page=239 |chapter-url=https://archive.org/details/in.ernet.dli.2015.104291/page/n283/mode/2up |access-date=2 May 2020 |chapter=Recent developments in electromagnetism}}</ref> The receiver in his apparatus consisted of a coil with a [[spark gap]], where a spark would be seen upon detection of electromagnetic waves. He placed the apparatus in a darkened box to see the spark better. However, he noticed that the maximum spark length was reduced when inside the box. A glass panel placed between the source of electromagnetic waves and the receiver absorbed ultraviolet radiation that assisted the electrons in jumping across the gap. When removed, the spark length would increase. He observed no decrease in spark length when he replaced the glass with quartz, as [[quartz]] does not absorb UV radiation.{{citation needed|date=November 2023}}

The discoveries by Hertz led to a series of investigations by [[Wilhelm Hallwachs]],<ref>{{Cite journal|last=Hallwachs|first=Wilhelm|date=1888|title=Ueber den Einfluss des Lichtes auf electrostatisch geladene Körper|journal=Annalen der Physik|language=en|volume=269|issue=2|pages=301–312|doi=10.1002/andp.18882690206|bibcode=1888AnP...269..301H|issn=1521-3889|url=https://zenodo.org/record/1423835}}</ref><ref name="Ref_g">Hallwachs, Wied. Ann. xxxiii. p. 301, 1888.</ref> Hoor,<ref name="Ref_h">Hoor, Repertorium des Physik, xxv. p. 91, 1889.</ref> [[Augusto Righi]]<ref name="Ref_i">Bighi, C. R. cvi. p. 1349; cvii. p. 559, 1888</ref> and [[Aleksandr Stoletov|Aleksander Stoletov]]<ref name="Ref_j">Stoletov. C. R. cvi. pp. 1149, 1593; cvii. p. 91; cviii. p. 1241; Physikalische Revue, Bd. i., 1892.</ref><ref name="Stoletov">
* {{cite journal |author=Stoletov, A. |year=1888 |title=Sur une sorte de courants electriques provoques par les rayons ultraviolets |journal=[[Comptes rendus de l'Académie des sciences|Comptes Rendus]] |volume=CVI |page=1149}} (Reprinted in {{cite journal|doi=10.1080/14786448808628270|title=On a kind of electric current produced by ultra-violet rays|year=1888|last1=Stoletov|first1=M.A.|journal=Philosophical Magazine |series=Series 5|volume=26|issue=160|page=317|url=https://zenodo.org/record/1431191}}; abstract in Beibl. Ann. d. Phys. 12, 605, 1888).
* {{cite journal |author=Stoletov, A. |year=1888 |title=Sur les courants actino-electriqies au travers deTair |journal=[[Comptes rendus de l'Académie des sciences|Comptes Rendus]] |volume=CVI |page=1593 }} (Abstract in Beibl. Ann. d. Phys. 12, 723, 1888).
* {{cite journal |author=Stoletov, A. |year=1888|title=Suite des recherches actino-electriques |journal=[[Comptes rendus de l'Académie des sciences|Comptes Rendus]] |volume=CVII |page=91}} (Abstract in Beibl. Ann. d. Phys. 12, 723, 1888).
* {{cite journal |author=Stoletov, A. |year=1889 |journal=[[Comptes rendus de l'Académie des sciences|Comptes Rendus]] |volume=CVIII |page=1241|title=Sur les phénomènes actino-électriques }}
* {{cite journal |author=Stoletov, A. |year=1889 |journal=Journal of the Russian Physico-chemical Society |volume=21 |page=159 |title=Актино-электрические исследовaния|language=ru}}
* {{cite journal |author=Stoletov, A. |year=1890 |journal=Journal de Physique |volume=9 |page=468|title=Sur les courants actino-électriques dans l'air raréfié|doi=10.1051/jphystap:018900090046800 |url=https://hal.archives-ouvertes.fr/jpa-00239138/document}}</ref> on the effect of light, and especially of ultraviolet light, on charged bodies. Hallwachs connected a zinc plate to an [[electroscope]]. He allowed ultraviolet light to fall on a freshly cleaned zinc plate and observed that the zinc plate became uncharged if initially negatively charged, positively charged if initially uncharged, and more positively charged if initially positively charged. From these observations he concluded that some negatively charged particles were emitted by the zinc plate when exposed to ultraviolet light. 

With regard to the ''Hertz effect'', the researchers from the start showed the complexity of the phenomenon of photoelectric fatigue—the progressive diminution of the effect observed upon fresh metallic surfaces. According to Hallwachs, [[ozone]] played an important part in the phenomenon,<ref name="Ref_m">{{cite journal|doi=10.1002/andp.19073280807|title=Über die lichtelektrische Ermüdung|year=1907|last1=Hallwachs|first1=W.|journal=Annalen der Physik|volume=328|issue=8|pages=459–516|bibcode = 1907AnP...328..459H |url=https://zenodo.org/record/1424105}}</ref> and the emission was influenced by oxidation, humidity, and the degree of polishing of the surface. It was at the time unclear whether fatigue is absent in a vacuum.{{citation needed|date=November 2023}}

In the period from 1888 until 1891, a detailed analysis of the photoeffect was performed by [[Aleksandr Stoletov]] with results reported in six publications.<ref name="Stoletov"/> Stoletov invented a new experimental setup which was more suitable for a quantitative analysis of the photoeffect. He discovered a direct proportionality between the intensity of light and the induced photoelectric current (the first law of photoeffect or [[Stoletov's law]]). He measured the dependence of the intensity of the photo electric current on the gas pressure, where he found the existence of an optimal gas pressure corresponding to a maximum [[photocurrent]]; this property was used for the creation of [[solar cell]]s.{{citation needed|date=October 2009}}

Many substances besides metals discharge negative electricity under the action of ultraviolet light. G. C. Schmidt<ref name="Ref_c">Schmidt, G. C. (1898) Wied. Ann. Uiv. p. 708.</ref> and O. Knoblauch<ref>{{Cite book|last=Knoblauch|first=O.|title=Zeitschrift für Physikalische Chemie|year=1899|volume=xxix|page=527}}</ref> compiled a list of these substances.

In 1897, [[J. J. Thomson]] investigated ultraviolet light in [[Geissler tube|Crookes tubes]].<ref name="Ref_o">''The International Year Book''. (1900). New York: Dodd, Mead & Company. p. 659.</ref> Thomson deduced that the ejected particles, which he called corpuscles, were of the same nature as [[cathode rays]]. These particles later became known as the electrons. Thomson enclosed a metal plate (a cathode) in a vacuum tube, and exposed it to high-frequency radiation.<ref>{{Cite book|title=Histories of the electron: the birth of microphysics|date=2001|publisher=MIT Press|author1=Buchwald, Jed Z. |author2=Warwick, Andrew.|isbn=978-0-262-26948-3|location=Cambridge, Mass.|oclc=62183406}}</ref> It was thought that the oscillating electromagnetic fields caused the atoms' field to resonate and, after reaching a certain amplitude, caused subatomic corpuscles to be emitted, and current to be detected. The amount of this current varied with the intensity and color of the radiation. Larger radiation intensity or frequency would produce more current.{{citation needed|date=October 2009}}

During the years 1886–1902, [[Wilhelm Hallwachs]] and [[Philipp Lenard]] investigated the phenomenon of photoelectric emission in detail. Lenard observed that a current flows through an evacuated glass tube enclosing two [[electrode]]s when ultraviolet radiation falls on one of them. As soon as ultraviolet radiation is stopped, the current also stops. This initiated the concept of photoelectric emission. The discovery of the ionization of gases by ultraviolet light was made by Philipp Lenard in 1900. As the effect was produced across several centimeters of air and yielded a greater number of positive ions than negative, it was natural to interpret the phenomenon, as J. J. Thomson did, as a ''Hertz effect'' upon the particles present in the gas.<ref name="Smithsonian report" />