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===Photoelectric effect=== The first demonstration of the [[photoelectric effect]] was carried out in 1887 by [[Heinrich Hertz]] using ultraviolet light.<ref>{{cite journal |author=H. Hertz |title=Ueber einen Einfluss des ultravioletten Lichtes auf die electrische Entladung |journal=Annalen der Physik |volume=267 |issue=8 |pages=983–1000 |year=1887 |url= https://books.google.com/books?id=79SWAAAAIAAJ&q=Annalen%20der%20Physik%20und%20Chemie%20hertz%201887&pg=PA983 |doi=10.1002/andp.18872670827|bibcode = 1887AnP...267..983H }}</ref> Significant for practical applications, Elster and Geitel two years later demonstrated the same effect using ''visible'' light striking alkali metals (potassium and sodium).<ref>{{cite journal|last1=Elster|first1=Julius|last2=Geitel|first2=Hans|title=Ueber die Entladung negativ electrischer Körper durch das Sonnen- und Tageslicht|journal=Annalen der Physik|volume=274|page=497|year=1889|doi=10.1002/andp.18892741202|bibcode = 1889AnP...274..497E|issue=12 |url=https://zenodo.org/record/1423862}}</ref> The addition of [[caesium]], another [[alkali metal]], has permitted the range of sensitive wavelengths to be extended towards longer wavelengths in the red portion of the visible spectrum. Historically, the photoelectric effect is associated with [[Albert Einstein]], who relied upon the phenomenon to establish the fundamental principle of [[quantum mechanics]] in 1905,<ref>{{cite journal |author=A. Einstein |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 |journal=Annalen der Physik |volume=322 |issue=6 |pages=132–148 |year=1905 |doi=10.1002/andp.19053220607 |bibcode=1905AnP...322..132E |url-status=live |archive-url=https://web.archive.org/web/20110709180735/http://www.physik.uni-augsburg.de/annalen/history/einstein-papers/1905_17_132-148.pdf |archive-date=2011-07-09 |doi-access=free }}</ref> an accomplishment for which Einstein received the 1921 [[Nobel Prize]]. It is worthwhile to note that Heinrich Hertz, working 18 years earlier, had not recognized that the kinetic energy of the emitted electrons is proportional to the frequency but independent of the optical intensity. This fact implied a discrete nature of light, i.e. the existence of ''quanta'', for the first time.
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