Note: Einstein's reasoning went as follows: Light shining on a metal may cause the emission of electrons (sometimes called photoelectrons in the context of the photoelectric effect). Since the energy of a wave is proportional to its intensity, one might expect that when the intensity of light shining on a metal is increased by increasing the number of photons, the kinetic energy of an emitted electron would increase accordingly. This is not the case. Instead, the kinetic energy of the electrons increases only when intensity is increased by increasing the frequency of each photon. If the frequency is less than the necessary quantum of energy, no electrons at all will be emitted regardless of the number of photons. This demonstrates that the electrons must be ejected by one-to-one photon-electron collisions rather than by the combined energies of two or more photons. It also shows that if a single photon does not have sufficient energy, no electron will be emitted.

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