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Compton vs photoelectric effect.

  1. Sep 7, 2013 #1
    i found a number of similar thread posing the same question but i didn't get the answer i was looking for, so i am asking it again.

    Basically, as far as i have understood photoelectric effect is about absorption of a photon by an electron and the extra energy converted to its kinetic energy; while in case of Compton effect there is collision (or scattering), not complete absorbtion, and the recoil.
    The two are about interaction of one photon-and-one electron, but with different energies of the photon, still in one, electron completely absorbs the photon and gets kinetic energy; while the other doesn't convert the whole energy to kinetic energy instead produces a photon of greater wavelength. How really is the electron able to decide when to absorb a photon and when to scatter it; how does the electron get to know the energy of the photon before interacting with it (i.e. absorbing); does it sense the energy and decides what to do next with it??
    What really happens??
    (please pardon me if the questions sound stupid and irrelevant).
  2. jcsd
  3. Sep 7, 2013 #2
    You can only have photoelectric effect if the electron is already in some potential. Say your electron is in some metal and you shine light on it to make it jump off - this is photoelectric effect.

    Compton scattering is scattering photons off free electrons.
  4. Sep 7, 2013 #3


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    If the electron is free, it cannot completely absorb the photon, because there is no final configuration with a single electron and no photon that conserves both energy and momentum. So a free electron can only undergo Compton scattering. The photoelectric effect requires that the electron be bound to something else (an atom or in a solid) so it can transfer the excess momentum.
  5. Sep 7, 2013 #4
    I agree to this.
    But Compton scattering can be observed by electrons of carbon atom, then why not for metals? Like photoelectric effect...
  6. Sep 7, 2013 #5


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    If the energy of the incident photon is very large compared to the binding energy (a free electron is of course just an extreme case of this) you get Compton scattering.

    If the energy of the incident photon is greater than than the binding energy, but not enormously so, you get the photoelectric effect.
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