Photoelectric effect

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.

The photoelectric effect is used for solar power by mounting arrays of solar
cells which generate direct current from sunlight.

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