Consider an isolated electron and a photon of energy 'hf' suffers collision with it. Then will the electron absorb all the energy of the photon or some amount is absorbed? If so, then does the photon after collision has less frequency than the previous one?

  • $\begingroup$ Answered here: physics.stackexchange.com/questions/358727/… - As @AnnaV has stated there, it is called Compton Scattering, "A photon interacts with the electron, the electron becomes off shell because it absorbs part of the four momentum of the incoming photon and a lower energy photon leaves". A full absorption is not possible without a violation of conservation of energy or momentum. $\endgroup$ – safesphere Oct 15 '17 at 3:08
  • $\begingroup$ "suffers collision with it" - what does 'suffers collision' mean? $\endgroup$ – Alfred Centauri Oct 15 '17 at 3:12

When a photon scatters off a free electron, it can either scatter elastically, i.e. no change in the frequency of the outgoing, or undergoes what is called Compton scattering. The distributions of the outgoing photonh can be calculated using the expansion in Feynman diagrams to first order :

Compton scattering

The outgoing photon will have a smaller frequency as the incoming transferred part of its momentum and energy to the electron. For the history see here.

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  • $\begingroup$ So complete absorption of energy doesn't take place by the electron. Right? $\endgroup$ – Gurbir Singh Oct 15 '17 at 5:50
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    $\begingroup$ Right, complete absorption will violate energy conservation in the center of mass, because the electron is an elementary particle ( no excited states) and its mass is fixed. $\endgroup$ – anna v Oct 15 '17 at 5:54
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    $\begingroup$ come to think of it also angular momentum conservation, as the spin1 of the photon cannot be accomodated by the electron at rest and alone. $\endgroup$ – anna v Oct 15 '17 at 7:43

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