The photoelectric effect is the flow of electric current in a material or through a vacuum (as in a photocell) when the material is exposed to light. Although the effect itself had been known for some time, Albert Einstein first described how it was caused by absorption of photons, or quanta of light, in the interaction of light with the electrons in the substance.
In the photoelectric effect, a metal plate is struck by light and emits electrons; the energy of those electrons is determined by the light's frequency, while the number of the electrons is determined by the light's intensity. This effect cannot easily be explained if light is assumed to be a wave.
The phenomenon was difficult to understand in terms of the classic wave description of light, as the energy of the emitted electrons did not depend on the intensity of the incoming light. Furthermore, for a given material, there would be a wavelength threshold: light longer than this wavelength, no matter what its intensity, did not produce the effect.
The simple explanation by Einstein in terms of absorption of single quanta of light explained the features of the phenomenon very well. The photons of the light beam have a characteristic energy given by the wavelength of the light. In the photoemission process, if an electron absorbs the energy of one photon and has more energy than the work function, it is ejected from the material. If the photon energy is too low, however, the electron is unable to escape the surface of the material. Increasing the intensity of the light beam does not change the energy of the constituent photons, only their number, and thus the energy of the emitted electrons does not depend on the intensity of the incoming light. It was for this insight that Einstein won his only Nobel Prize.