I was reading photoelectric effect , which was completely explained by Einstein. And a bit difficult question , atleast seems to me is whether photoelectric effect is inelastic collision or elastic collision . Every where it is written that photoelectric effect is example of inelastic collision . But as far as I know in inelastic collision some energy become internal energy of the system , there maybe some energy loss as heat or something . But here in photoelectric effect photon is being absorbed by electron but where is the energy loss? No energy is being lost here the energy $h\nu $ is absorbed by electron and its kinetic energy increases so no energy loss occurs . So in my opinion it is not an inelastic collision . So what is the reality , is it inelastic or elastic collision ?
When a photon interacts with an atom, three things can happen:
elastic scattering, when the photon keeps its energy, and changes angle
inelastic scattering, when the photon gives part of its energy to the atomic system and changes angle
absorption, when the photon gives all its energy to the atom, and the valence electron will move to a higher energy level, and then the electron will eventually move back to a lower energy level and emit a photon
At low energies, Rayleigh scattering happens, 1. elastic.
In your case, photo electricity is when 3. happens. That is at higher energies.
At even higher energies, 2. happens, Compton scattering.