The Photoelectric Effect

Einstein in 1905 used Planck's idea of light quanta, or photons, to explain the photoelectric effect, wherein blue light shined on negatively charged metal would cause the charge to leak away. For an explanation, please see http://www.cmmp.ucl.ac.uk/~ajf/1B23/qm1/node3.html

This showed that Planck's theory might be far more important than simply explaining one phenomenon, incandescence. Light behaved like a particle in addition to a wave; quantum physics was born.

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