The Photoelectric Effect

The photoelectric effect is a quantum-mechanical phenomenon in which light impinging on the surface of a metal can cause electrons to be ejected. Only light with wavelengths shorter than some threshold value , characteristic of each metal, can cause emission of photoelectrons, no matter how intense the radiation. Einstein explained this in 1905 by proposing that light is composed of discrete photons, each carrying energy . Only when the photon energy exceeds the work function of the metal, a measure of how strongly the outermost electrons are bound, can photoelectrons be emitted. The relevant equation is , where the last term represents the maximum kinetic energy of the ejected electrons. Once the threshold wavelength is attained, the current of electrons increases linearly with the radiation intensity. This can be monitored by an ammeter in the circuit shown. The light source covers the entire visible range 400–700 nm. In the ultraviolet region, the light ray appears as black.
Einstein was awarded the 1922 Nobel Prize in Physics for his theory of the photoelectric effect, rather than for his discoveries of the special and general theories of relativity.


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Snapshot 1: above the threshold wavelength, there is no emission of photoelectrons, no matter how high the radiation intensity
Snapshot 2: at the threshold wavelength for Ag, around 290 nm, the photoelectric effect occurs
Snapshot 3: once the threshold is reached, the electron current increases linearly with radiation intensity
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