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Photoelectric Effect and QED

  1. Sep 21, 2016 #1
    I've read that the photoelectric effect cannot be explained according to Maxwell's equations and theory of electromagnetism. Classical EM theory treats light as a wave and states that the intensity of the light matters and not the frequency. According to Einstein, if we think of light as a particle instead of a wave that we can predict and explain the photoelectric effect. Why is this so? Why can't we use classical EM to explain the photoelectric effect? How does changing from a wave view to a particle view of light help explain the photoelectric effect?

    Do other theories like QED also accurately explain the effect?
  2. jcsd
  3. Sep 21, 2016 #2

    A. Neumaier

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    We cannot use classical electrodynamics alone.

    But we can use classical electrodynamics together with quantum electrons to explain the photoelectric effect. The quantized electrons form a bistable system (each electron can be either bound or free), and the electromagnetic field excites some of them, because of quantum mechanics on a random basis, with details that match the observed effects: A minimal frequency (the ionization energy) is needed to produce the effect. If this is the case, the mean rate of electron production is proportional to the intensity of the incident field.

    A particle view for light is not needed for the photoeffect. Photons are relevant only to explain other quantum effects such as parametric downconversion, which is the basis for entanglement experiments.
  4. Sep 21, 2016 #3
    Light is fascinating in that it is both a particle and a wave. Sounds impossible, right? While theories designed to reconcile existing theories (relativity, Maxwell's equations, etc) with quantum physics have hit major roadblocks, we know that light has quantum properties because of the double slit experiment. When light is directed through two slits in a board, it will leave one of two patterns on photosensitive paper- either two "shadows" demonstrating behavior as a particle, or a window blind "shadow" which demonstrates light's behavior as a wave. The interesting thing about this experiment is that it initially caused quite a bit of controversy, with multiple scientists getting different results with essentially the same process. It was eventually discovered that the mere act of observing the experiment (or not) determined whether light behaved as a wave or a particle.
  5. Sep 21, 2016 #4
  6. Sep 21, 2016 #5


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    There are two things involved here:

    1. The understanding of wave mechanics. More importantly, the fact that the ENERGY of a wave depends on its amplitude, and thus, increasing the amplitude also increases the intensity. More intense, more energy. Yet, the most energetic photoelectrons emitted are not dependent on the change in this intensity. So the wave picture is not compatible with experiment. The Einstein model has a simple and very clear explanation to account for this without too much gymnastics.

    2. While the photoelectric effect is still predominantly used as a evidence for the photon picture, recent theoretical development in stochastic electrodynamics (SED) have claimed that they can match the observation of photoelectric effect using classical E&M. While I'm still not convinced on this, and while the majority of physics accepts the photon picture explanation for photoemission, it leaves the fact that the photoelectric effect alone cannot be used to falsify classical E&M.

    I am sure Greg is paging me on this to prod me to finish my photoelectric effect article for Insight! :)

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