The process in which visible light, x rays or gamma rays incident on matter, cause an electron to be ejected. The ejected electron is called a photoelectron.

The photoelectric effect was discovered by Heinrich Hertz in 1897 while performing experiments that led to the discovery of electromagnetic waves. Since this was just about the time that the electron itself was first identified the phenomenon was not really understood. It soon became clear in the next few years that the particles emitted in the photoelectric effect were indeed electrons. The number of electrons emitted depended on the intensity of the light but the energy of the photoelectrons did not. No matter how weak the light source was made the maximum kinetic energy of these electrons stayed the same. The energy however was found to be directly proportional to the frequency of the light. The other perplexing fact was...