| 单词 | photoelectric effect |
| 释义 | photoelectric effect The liberation of electrons (see photoelectron) from a substance exposed to electromagnetic radiation. The number of electrons emitted depends on the intensity of the radiation. The kinetic energy of the electrons emitted depends on the frequency of the radiation. The effect is a quantum process in which the radiation is regarded as a stream of photons, each having an energy hf, where h is the Planck constant and f is the frequency of the radiation. A photon can only eject an electron if the photon energy exceeds the work function, φ, of the solid, i.e. if hf0 = φ an electron will be ejected; f0 is the minimum frequency (or threshold frequency ) at which ejection will occur. For many solids the photoelectric effect occurs at ultraviolet frequencies or above, but for some materials (having low work functions) it occurs with light. The maximum kinetic energy, Em, of the photoelectron is given by Einstein's equation: Em = hf - φ (see also photoionization).Apart from the liberation of electrons from atoms, other phenomena are also referred to as photoelectric effects. These are the photoconductive effect and the photovoltaic effect . In the photoconductive effect, an increase in the electrical conductivity of a semiconductor is caused by radiation as a result of the excitation of additional free charge carriers by the incident photons. Photoconductive cells , using such photosensitive materials as cadmium sulphide, are widely used as radiation detectors and light switches (e.g. to switch on street lighting). In the photovoltaic effect, an e.m.f. is produced between two layers of different materials as a result of irradiation. The effect is made use of in photovoltaic cells , most of which consist of p—n\nsemiconductor junctions (see also photodiode; phototransistor). When photons are absorbed near a p—n junction new free charge carriers are produced (as in photoconductivity); however, in the photovoltaic effect the electric field in the junction region causes the new charge carriers to move, creating a flow of current in an external circuit without the need for a battery. See also photoelectric cell |
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