Light, and all other forms of electromagnetic radiation, is emitted in discrete ‘packet’ of energy. Each ‘packet’ was called a quantum of energy (or photon).
When an atom emits radiation, its energy changes by certain allowed amounts only. (More information in the later sub-topics)
$E = hf$, where E is energy of a single photon, h is Planck constant, f is frequency
When light is directed at a metal surface,
- A stream of photon bombards the surface of metal
- Any free electron near the surface could be struck by a photon and gains energy
- If the gain in energy is sufficient to overcome the electromagnetic force of attraction between the electrons and the positive nucleus, the electron can leave the plate
- Each photoelectron from a metal plate has gained the whole amount of energy of a single photon. The photon will be reflected or transmitted, if not absorbed. The photon’s energy cannot be shared among the electrons – it must give up all its energy to a single electron.