Photoelectric Effect

Photoelectric Effect, liberation of electrically charged particles from matter when light or other electromagnetic radiation falls on it. The term photoelectric effect describes several types of related interactions. In the external photoelectric effect, electrons are liberated from the surface of a metallic conductor by absorbing energy from light shining on the metal’s surface.

Study of the external photoelectric effect played an important role in the development of modern physics. Experiments beginning in 1887 showed that the external photoelectric effect had certain qualities that could not be explained by the theories of that time, in which light and all other kinds of electromagnetic radiation were considered to behave like waves. For example, as the light shining on a metal becomes increasingly intense, the classical wave theory of light suggests that the electrons that absorb the light will be liberated from the metal with more and more energy. However, experiments showed that the maximum possible energy of the ejected electrons depends only on the frequency of the incident light, and not on its intensity.

In 1905, in an effort to explain how the external photoelectric effect occurs, Albert Einstein suggested that light could be considered to behave like particles in some instances, and that the energy of each light particle, or photon, depends only on the wavelength of the light. To explain the external photoelectric effect, he envisioned light as a collection of projectiles hitting the metal. A free electron in the metal that is struck by a photon absorbs the photon’s energy. If the photon is sufficiently energetic, the electron is dislodged from the metal. Einstein’s theory explained many features of the external photoelectric effect, such as why the maximum energy of the ejected electrons is independent of the intensity of the incident light. According to his theory, the maximum energy of a dislodged electron depends only on the energy of the photon that ejects the electron, which depends only on the light’s wavelength, or frequency. Einstein’s photoelectric equation expresses these ideas mathematically:

where h is Planck’s constant, f is the frequency of the incident light, and ym_ev²_m is the maximum kinetic energy of the photoelectron. The quantity φ is the work function of the metal surface under consideration, which is the energy required to completely remove an electron from the metal.

Einstein’s theory was later verified through further experimentation. His explanation of the photoelectric effect, with its demonstration that electromagnetic radiation can behave like a collection of particles in some situations, contributed to the development of quantum theory.

The external photoelectric effect is applied in the photomultiplier tube, in which a single incident photon produces a pulse of current through an external circuit by means of a series of electron avalanches. Photomultiplier tubes are used in image-intensifying cameras to obtain pictures in extremely low light levels, and in scintillation cameras used to detect ionizing events. In the case of the scintillation counter, the photon of light comes from a crystal of sodium iodide that emits a weak flash of light when ionizing radiation passes through it.

The term photoelectric effect can also refer to three other processes: photoionization, photoconduction, and the photovoltanic effect. Photoionization is the ionization of a gas by light or other electromagnetic radiation; the photons must possess enough energy to detach one or more outer electrons from the gas atoms. In photoconduction, electrons in crystalline matter, by absorbing energy from photons, are brought to the range of energy levels at which they can move freely to conduct electricity. In the photovoltanic effect, photons create electron-hole pairs in semiconducting materials (see Semiconductor). In a transistor, this effect causes the formation of an electric potential across the junction between two different semiconductors.