Photoelectric effect

The photoelectric effect is the emission of <a href="/facts/Electron/L0KBo5cW">electrons</a> from a material caused by <a href="/facts/Electromagnetic_radiation/Ynx7ujCW">electromagnetic radiation</a> such as <a href="/facts/Ultraviolet_light/kncBUqn5">ultraviolet light</a>. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in <a href="/facts/Condensed_matter_physics/EBVI5nmL">condensed matter physics</a>, <a href="/facts/Solid-state_chemistry/MFpqNGSt">solid state</a>, and <a href="/facts/Quantum_chemistry/H9AXV0lx">quantum chemistry</a> to draw inferences about the properties of atoms, molecules and solids. The effect has found use in <a href="/facts/Electronics/E4JhgISq">electronic devices</a> specialized for light detection and precisely timed electron emission.
The experimental results disagree with <a href="/facts/Classical_electromagnetism/vK0YI6dj">classical electromagnetism</a>, which predicts that continuous light waves transfer <a href="/facts/Energy/wd8PrQM7">energy</a> to electrons, which would then be emitted when they accumulate enough energy. An alteration in the <a href="/facts/Intensity_(physics)/KFWkJkJI">intensity</a> of light would theoretically change the <a href="/facts/Kinetic_energy/aJRxUd4p">kinetic energy</a> of the emitted electrons, with sufficiently dim light resulting in a delayed emission. The experimental results instead show that electrons are dislodged only when the light exceeds a certain <a href="/facts/Frequency/QUSbPaW8">frequency</a>—regardless of the light's intensity or duration of exposure. Because a low-frequency beam at a high intensity does not build up the energy required to produce photoelectrons, as would be the case if light's energy accumulated over time from a continuous wave, <a href="/facts/Albert_Einstein/CNTet8qu">Albert Einstein</a> proposed that a beam of light is not <a href="/facts/Light/jeofpMn4">a wave</a> propagating through space, but discrete energy packets, which were later popularised as <a href="/facts/Photon/mFNhAEzA">photons</a> by <a href="/facts/Gilbert_N._Lewis/wWnV6QXL">Gilbert N. Lewis</a> since he coined the term 'photon' in his letter "The Conservation of Photons" to Nature published in 18 December 1926.
Emission of conduction electrons from typical <a href="/facts/Metal/PT8hNX6R">metals</a> requires a few <a href="/facts/Electronvolt/YMT1fqX9">electron-volt</a> (eV) light quanta, corresponding to short-wavelength visible or ultraviolet light. In extreme cases, emissions are induced with photons approaching zero energy, like in systems with negative electron affinity and the emission from excited states, or a few hundred keV photons for <a href="/facts/Core_electron/LQuGLKcG">core electrons</a> in <a href="/facts/Chemical_element/0jducgWD">elements</a> with a high <a href="/facts/Atomic_number/bgAr581S">atomic number</a>. Study of the photoelectric effect led to important steps in understanding the quantum nature of light and electrons and influenced the formation of the concept of <a href="/facts/Wave%25E2%2580%2593particle_duality/Au3UlcwQ">wave–particle duality</a>. Other phenomena where light affects the movement of electric charges include the <a href="/facts/Photoconductivity/kPVR5C44">photoconductive</a> effect, the <a href="/facts/Photovoltaic_effect/huqzdkH2">photovoltaic effect</a>, and the <a href="/facts/Photoelectrochemical_cell/Pq2uYSNI">photoelectrochemical effect</a>.

Photoelectric effect open-in-new

Photoelectric effect