Suppose there is light of high enough frequency so it causes photoelectric effect (in a piece of metal). Does such light cause photoelectric effect on free electrons as well?
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2$\begingroup$ For free electrons, you have the Compton scattering process. $\endgroup$– Count IblisCommented Mar 20, 2015 at 16:26
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1$\begingroup$ @CountIblis: that should be an answer $\endgroup$– John RennieCommented Mar 20, 2015 at 16:41
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1$\begingroup$ "The photoelectric effect is the observation that many metals emit electrons when light shines upon them." - How is it meaningful to ask whether this effect happens for free electrons, when it, by definition, happens in metals? $\endgroup$– ACuriousMind ♦Commented Mar 20, 2015 at 23:42
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2$\begingroup$ @ACuriousMind: a sympathetic interpretation would be to ask if light has any interaction with free electrons that is analogous to the photoelectic effect, and this seems to me a reasonable if rather basic question. $\endgroup$– John RennieCommented Mar 21, 2015 at 11:35
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3 Answers
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The photoelectric effect occurs when an atom (or ion) absorbs the energy of a photon and an electron is emitted. Some of the photon energy is used to liberate the electron, the rest goes into the electron kinetic energy.
The same thing cannot happen when light interacts with a free electron (e.g. see Free electron can't absorb a photon ). A simple proof shows that a free electron cannot absorb all the energy of a photon and simultaneously conserve linear momentum.
Instead, part of the photon energy is given to the electron and a photon of lower energy is scattered. This is known as the Compton effect.
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$\begingroup$ How would such a simple proof look like? It doesn't seem obvious to me.. :( $\endgroup$– BenCommented Dec 12, 2017 at 21:27
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1$\begingroup$ @Ben physics.stackexchange.com/questions/225522/… $\endgroup$– ProfRobCommented Dec 12, 2017 at 23:49
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The photoelectric effect - ejection of electrons from matter due to light - is a phenomenon that occurs when light shines on a piece of metal. I do not think such phenomenon was observed with free electrons in vacuo.
( Compton scattering is often described as if electrons interacting with the light were free, but it actually refers to an experiment where the electrons are bound to nuclei in matter. )
answered Mar 20, 2015 at 23:06
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We can prove that photoelectric effect does not happen for free electron.
we look at the hypothetical process in the center-of-mass system, which is defined as that system in which the initial momentum is zero. From the conservation of energy. E initial = E final
hv +Mc^2 = Mo c^2
( Mo is the rest mass) ( M is the relativistic mass)
or
which implies Mo> M Since this cannot be true the process can't occur.
The electrons participating in the photoelectric effect are not free. The heavy matter present takes off momentum but absorbs a negligible amount of energy.
