Photoelectric Effect

The photoelectric effect is the emission of electrons from a material when it absorbs light or other electromagnetic radiation of sufficient photon energy. The emitted electrons are called photoelectrons. In its usual, narrow meaning, the term describes electrons escaping a surface, rather than merely becoming mobile inside a material. Its explanation connected the energy of individual light quanta to electron emission and became an important foundation of quantum mechanics. (openstax.org)

Discovery and historical development

Heinrich Hertz first observed the effect in 1887. Subsequent investigations, notably those of Philipp Lenard, examined the properties of the emitted electrons using electrical measurements. These experiments established that light could release charged particles from illuminated surfaces and provided evidence that the emission process did not follow a simple classical account of energy absorption. (nobelprize.org)

In 1905, Albert Einstein explained photoelectric emission by proposing that light transfers energy in discrete quanta, now called photons. His argument extended Max Planck’s treatment of blackbody radiation: rather than restricting quantization to exchanges between matter and radiation, Einstein attributed discrete energy to light itself. This was a major step in the development of energy quantization as a general physical principle. (openstax.org)

Einstein’s 1921 Nobel Prize in Physics, awarded in 1922, specifically recognized his discovery of the law of the photoelectric effect, alongside his services to theoretical physics. The distinction concerned the photoelectric law, not his theory of relativity. (nobelprize.org)

Experimental characteristics

A standard experiment illuminates an emitting electrode inside an evacuated tube. A second electrode collects the emitted electrons, producing a photocurrent. An adjustable voltage can either assist collection or oppose the electrons’ motion. The vacuum reduces collisions that would otherwise alter their energies before detection. (openstax.org)

Three observations are central:

• Threshold frequency: In ordinary single-photon emission, a given surface has a minimum incident frequency below which electrons are not emitted.
• Frequency dependence: Above threshold, the maximum electron kinetic energy increases with light frequency.
• Intensity dependence: At fixed frequency, stronger illumination generally increases photocurrent without increasing the maximum electron kinetic energy. Emission begins promptly, without the prolonged energy-accumulation delay expected from a simple classical model. (openstax.org)

These results challenged an explanation based solely on continuous energy delivery by waves. In that picture, increasing intensity should increase the energy available to an electron, and sufficiently prolonged illumination should eventually enable escape even at low frequency. The observed threshold and frequency-dependent electron energies instead required a different account of absorption. This did not invalidate classical electromagnetism for phenomena such as wave propagation; it identified limitations in its description of light–matter energy exchange. (openstax.org)

Einstein’s photoelectric equation

A photon of frequency $\nu$ carries energy

where $h$ is the Planck constant, $c$ is the speed of light in vacuum, and $\lambda$ is the vacuum wavelength. In the elementary single-photon model, an electron absorbs one photon. Part of that energy enables escape; the remainder appears as kinetic energy. (openstax.org)

For a metal surface, Einstein’s equation is

Here $K_{\max}$ is the maximum kinetic energy of emitted electrons, and $\phi$, the work function, is the minimum energy required to remove an electron from the metal. The threshold frequency and corresponding longest wavelength are therefore

The equation concerns the maximum energy, not an identical energy for every emitted electron. Photoelectrons can emerge with lower energies; the experiment consequently measures an energy distribution with an upper limit. (openstax.org)

An opposing voltage can suppress collection. If $V_s$ denotes the positive magnitude of the stopping potential, then

where $e$ is the magnitude of the electron’s electric charge. Thus a graph of stopping potential against frequency has slope $h/e$. Measurements connect an electrical observable directly to the quantum relation between frequency and energy. (openstax.org)

Related light-induced electrical effects

External photoelectric emission should be distinguished from the photovoltaic effect. In a photovoltaic device, absorbed light supplies energy to charge carriers within a semiconductor, allowing electrical current to be extracted through contacts. The operating principle does not require electrons to escape into vacuum. A solar cell therefore converts radiation into electrical power through carrier excitation and transport inside a material, rather than the surface-emission arrangement used in the classic experiment. (energy.gov)

Detection and spectroscopy

The external effect is used in photomultiplier tubes. Incident photons strike a photocathode, releasing electrons into vacuum. These electrons travel through successive multiplying electrodes, called dynodes, where secondary emission amplifies the signal before collection at an anode. Photocathode composition and the transmission properties of the entrance window determine the accessible wavelength range. (hamamatsu.com)

A detector’s quantum efficiency is the ratio of emitted photoelectrons to incident photons at a specified wavelength. It distinguishes the photon energy required for emission from the probability that an incident photon actually produces an escaping electron. (hamamatsu.com)

The effect also underlies photoelectron spectroscopy, a branch of spectroscopy that examines the energies of electrons released by radiation. Measuring those energies reveals information about a material’s electronic structure. Angle-resolved photoemission spectroscopy additionally measures emission directions, enabling investigation of electronic band structure in condensed matter physics. X-ray-based variants can probe electronic properties beneath the immediate surface. (newscenter.lbl.gov)