Photoelectric Effect

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The photoelectric effect is the emission of electrons from a metal plate when light shines on it. One might think that if one made the light brighter (higher intensity, or more energy) then higher energy electrons would be ejected, and if the intensity were too low, then none would be. However, early experiments suggested that red light, no matter how bright, would not eject any electrons, while blue light, even very low intensity, would still eject electrons. The "bluer" the light, the higher the energy of the ejected electrons. This led Einstein to postulate the particle nature of light, where the color (or frequency) determined the energy of the light particle (photon), and the intensity only determined the number of photons in the beam. If a photon did not have enough energy (was too red), electrons would not be ejected, no matter how many photons there were. In the full quantum theory, however, the most important part is actually the quantized energy levels of the atoms in the metal plate. (Unit: 2)