The question of whether light was a particle or a wave is an old one. Newton thought light was made up of particles, Young that it was made up of waves. The argument seemed to be settled in favour of the wave theory until the photoelectric effect and the birth of quantum physics.

The photoelectric effect seemed to contradict that light could be a wave. If light were a wave, an electron close to the surface of a metal could gradually absorb energy and would eventually have enough energy to escape the surface of the metal. This would happen whatever the intensity or frequency of the light incident on the metal. The photoelectric effect clearly demonstrated that no electrons at all would be emitted if the frequency of the incident light were below a certain level, called the threshold frequency.

The photoelectric effect demanded an explanation that involved light not behaving as a wave. Einstein provided it. His explanation was