Photoelectric effect

Albert Einstein did not win the Nobel Prize for his Special Theory of Relativity or the General Theory of Relativity, but for his explanation of the workings of the photoelectric effect (PE).

With this effect, certain frequencies of light shone onto a copper plate cause the plate to eject electrons. He suggested that these packets of light or quanta (now called photons) could explain the PE. High-frequency light, such as blue or ultraviolet light, could cause electrons to be ejected—but not low-frequency red light which did not lead to electron ejection. The energy of individual emitted electrons actually increases with the frequency (and, hence, the color) of the light. Rather than light exerting its effect as a classical wave, Einstein suggested that the energy of light came in the form of packets, or quanta, and that this energy was equal to the light frequency multiplied by Planck’s constant. If the photon was below a threshold frequency, it just did not have the energy to kick out an electron.

Einstein’s explanation for the energy of the photons seemed to account for many observations, such as for a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons can be emitted. Today, numerous devices, such as solar cells, rely on conversion of light to electric current in order to generate power.

Studies of the statistical properties of the photons in the 1970s provided experimental verification of the manifestly quantum (nonclassical) nature of the electromagnetic field.