# 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?

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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Simon Zhang
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.