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 (i.e., a high frequency electromagnetic wave). The photoelectric effect helped further the idea of wave-particle duality, the concept that physical systems can display both wave-like and particle-like properties, and that was used as a fundamental principle by the creators of quantum mechanics. A complete picture of the photoelectric effect was only obtained after the maturity of quantum mechanics.
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. For a given material, there would be a wavelength threshold: radiant light energy longer than this wavelength, no matter what its intensity, did not produce the effect. 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.
In 1901 on November 5, Nikola Tesla recieved the patent US685957 (Apparatus for the Utilization of Radiant Energy) that describes radiation charging and discharging conductors (ex., a metal plate) by "radiant energy". Radiant energy throws off with great velocity minute particles (i.e., "electrons") which are strongly electrified. Rays or radiation falling on insulated-conductor connected to a condenser (i.e., a capacitor), the condenser indefinitely charges electrically. The patent specified that the radiation (or radiant energy) include many different forms. These devices have been refered to as "Photoelectric alternating current stepping motors".
Although the effect itself had been described earlier by Nikola Tesla in the patent US685957, Albert Einstein's simple description (in 1905) on how it was caused by absorption of photons, or quanta of light, in the interaction of light with the electrons in the substance helped him win the Nobel Prize. The paper, named "On a Heuristic Viewpoint Concerning the Production and Transformation of Light", proposed the simple description of "light quanta" (now called photons) and showed how they could be used to explain such phenomena as the photoelectric effect. The simple explanation by Einstein in terms of absorption of single quanta of light explained the features of the phenomenon and helped explained the characteristic energy.
The idea of light quanta was motivated by Max Planck's earlier derivation of the law of blackbody radiation by assuming that luminous energy could only be absorbed or emitted in discrete amounts, called quanta. Einstein showed that, by assuming that light actually consisted of discrete packets, he could explain the photoelectric effect. The idea of light quanta contradicted the wave theory of light that followed naturally from James Clerk Maxwell's equations for electromagnetic behavior and, more generally, the assumption of infinite divisibility of energy in physical systems. Even after experiments showed that Einstein's equations for the photoelectric effect were accurate, his explanation was not universally accepted.
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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