Explain how the photoelectric effect gives evidence for the photon theory of light.

When light is shone onto a piece of metal, if the light is below a certain frequency, no photoelectrons are emitted. Only if the light is above a certain frequency, known as the threshold frequency, are photoelectrons emitted. This can't be explained using the wave theory of light. If light behaved only as a wave, eventually light of any frequency would deliver enough energy to the electrons of the metal for them to escape. However, to explain the photoelectric effect, we imagine light as packets of energy known as photons, where one photon can only transfer its energy to one electron, and if the photon has enough energy to give to the electron, it will be emitted. If the photon doesn't have enough energy, no electrons will be emitted. The minimum energy required by a photon to liberate photoelectrons is known as the work function and depends upon the metal used. The energy of a photon is given by E=hf (where h is Planck's constant, and f is the frequency of the light). Therefore, only photons of a high enough frequency will have energy greater than the work function, and so only light of a high enough frequency will liberate photoelectrons, demonstrating that in the photoelectric effect light must display particle-like behaviour.