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The Photoelectric Effect

The photoelectric effect is that when a beam of light shines on certain metal surfaces, a beam of electrons is produced.  The photoelectric effect cannot be explained by classical physics, which would predict that the kinetic energies of the electrons leaving the surface should depend of the brightness of the light.  The facts are that the kinetic energies depend on the frequency of the light, not the brightness.  Dim blue light produces electrons with higher energies than does bright red light.  Also if the light is below a certain minimum frequency, called the threshold frequency, the photoelectric effect is not not observed at all.  

Albert Einstein postulated that light consists of quanta (photons), or particles of electromagnetic energy, with energy E proportional to the frequency of the light,  E = .  In 1905 Einstein used this quantum concept to explain the photoelectric effect.







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