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=== Evidence of quanta from the photoelectric effect ===
{{main | Photoelectric Effect}}
The seeds of the quantum revolution appear in the discovery by [[JJ Thomson]] in 1897 that [[cathode rays]] were not continuous but "corpuscles" identical to [[electrons]]. Electrons had been named just six years ealier as part of the emerging theory of [[atoms]]. In 1900, [[Max Planck]], a conservative physicist unconvinced by the [[atomic theory]], discovered that he needed discrete entities like atoms or electrons to explain [[blackbody radiation]].<ref name=Baggott>{{Cite book |last=Baggott |first=J. E. |title=The quantum story: a history in 40 moments |date=2013 |publisher=Oxford Univ. Press |isbn=978-0-19-965597-7 |edition=Impression: 3 |___location=Oxford}}</ref>
[[File:Black body.svg|thumb|upright=1.4|Blackbody radiation intensity vs color and temperature. The rainbow bar represents visible light; 5000K objects are "white hot" by mixing differing colors of visible light. To the right is the invisible infrared. Classical theory (black curve for 5000K) fails; the other curves are correct predicted by quantum theories.]]
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At the time, electrons, atoms, and discrete oscillators were all exotic ideas to explain exotic phenomena. But in 1905 [[Albert Einstein]] proposed that light was also corpuscular, consisting of "energy quanta", seemingly in contradiction to the established science of light as a continuous wave, stretching back a hundred years to [[Thomas Young]]'s work on [[diffraction]].
His revolutionary proposal started by reanalyzing Planck blackbody theory, arriving at the same conclusions by using the new "energy quanta". Einstein then showed how energy quanta connected to JJ Thomson's electron. In 1902, [[Philipp Lenard]] directed light from an arc lamp onto freshly cleaned metal plates housed in an evacuated glass tube. He measured the electric current coming off the metal plate, for higher and lower intensity of light and for different metals. This is the [[photoelectric effect]]. Lenard showed that amount of current – the number of electrons – depended on the intensity of the light, but that the velocity of these electrons did not depend on intensity. The continuous wave theories of the time would predict that more light intensity would accelerate the same amount of current to higher velocity contrary to experiment. Einstein's energy quanta explained the volume increase: one electron is ejected for each quanta: more quanta mean more electrons.<ref name=Baggott/>{{rp|23}}
Einstein then predicted that the electron velocity would increase in direct proportion to the light frequency above a fixed value that depended upon the metal. Here the idea is that energy in energy-quanta depends upon the light frequency; the energy transferred to the electron comes in proportion to the light frequency. The type of metal gives a [[work function | barrier]], the fixed value, that the electrons must climb over to exit their atoms, to be emitted from the metal surface and be measured.
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