Consider a beam of electrons (each electron with energy E₀) incident on a metal surface kept in an evacuated chamber. Then Chapter Eleven

When a beam of electrons (each with energy E₀) is incident on a metal surface, electrons can be emitted as there will be collision between the electrons.

If the collision is elastic, electrons are emitted with energy E₀.

If the collision is inelastic, some energy is used in liberating the electrons from the metal surface and electrons are emitted with energy less than E₀.

Hence, maximum energy of liberated electrons is E₀.