User:Johnjbarton/sandbox/introduction to quantum mechanics: Difference between revisions

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=== Evidence of quanta from the photoelectric effect ===

The seeds of the quantum revolution appear in the discovery by [[JJ Thomson]] that [[cathode rays]] were actually "corpuscles" or particles now called [[electrons]]. Since no solid theory of cathode rays existed, the electron was exciting news, but not a revolution. However, in 1905 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| Young's]] work on [[diffraction]]. Light quanta would be revolutionary.

[[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. Curves are predicted by quantum theories.]]

Einstein's evidence was twofold. First he analyzed [[blackbody radiation]]. Hot objects radiate heat; very hot objects – red hot, white hot objects – all look similar when heated to the same temperature. This temperature dependent "look" results from a common curve of light intensity at different frequencies (colors). The common curve is called blackbody radiation. The lowest frequencies are invisible heat rays – infrared light. White hot objects have intensity across many colors in the visible range. Continuous wave theories of light and matter cannot explain the blackbody radiation curve. Einstein showed that, by assuming that light energy transferred in discrete "energy quanta", the radiation curve could be explained. [[Max Planck]] showed the same result five years earlier, but he did not propose that the light was quantized.