A photon is an elementary particle representing a quantum of electromagnetic radiation, including visible light, radio waves, and gamma rays. In quantum field theory, it is an excitation of the electromagnetic field. Within the Standard Model, photons mediate electromagnetic interactions. They have zero rest mass and no electric charge, yet carry energy and momentum. Their behavior combines wave-like propagation with discrete exchanges of energy, making them central to the quantum description of radiation. (home.web.cern.ch)
Historical development
The photon concept emerged from difficulties in explaining radiation using classical physics alone. In 1900, Max Planck introduced energy quantization in his treatment of blackbody radiation, relating energy increments to radiation frequency. In 1905, Albert Einstein proposed that light itself could behave as discrete, localized energy quanta. This hypothesis explained important features of the photoelectric effect, in which illumination ejects electrons from a material. Einstein received the 1921 Nobel Prize in Physics, awarded in 1922, especially for discovering the law governing that effect. (nobelprize.org)
Evidence strengthened through Arthur Compton’s 1923 work on Compton scattering. The wavelength change of scattered X-rays could be explained by treating the interaction as a collision between a photon and an electron, conserving energy and momentum. Gilbert N. Lewis introduced the term photon in his 1926 paper “The Conservation of Photons.” The modern field-theoretic description subsequently connected the particle properties of radiation with its wave behavior. (nobelprize.org)
Energy, momentum, and polarization
For a photon of definite frequency, its energy is
[ E=h\nu=\frac{hc}{\lambda}, ]
where (h) is the Planck constant, (\nu) is frequency, (c) is the speed of light in vacuum, and (\lambda) is vacuum wavelength. Higher-frequency photons therefore carry more energy. The electromagnetic spectrum classifies radiation by frequency or wavelength; its regions do not represent different kinds of elementary particle. Radio photons and gamma-ray photons differ principally in energy, not in their fundamental identity. (imagine.gsfc.nasa.gov)
Photons propagate through vacuum at (c), approximately (299{,}792{,}458) metres per second. Zero rest mass does not imply zero energy or momentum: momentum transfer is directly involved in Compton scattering. The energy of an individual photon should also be distinguished from the total energy of a light pulse, which can contain many photons. (imagine.gsfc.nasa.gov)
A photon is a spin-1 boson. Its intrinsic spin angular momentum is associated with polarization. For a specified propagation direction, freely propagating light has two independent transverse polarization components. Horizontal and vertical linear polarization provide one possible basis; general polarization states can be expressed as superpositions of these basis states. As bosons, photons can occupy the same quantum state, unlike particles governed by the exclusion principle. (atlas-public.web.cern.ch)
Wave behavior and quantum measurement
Photons are not adequately described as tiny classical objects following predetermined paths. Quantum mechanics instead describes possible measurement outcomes through quantum states. Wave-like interference remains observable even when photons pass through an apparatus individually. In a double-slit experiment, separate detections gradually build up an interference pattern, although each registered event is localized. Experiments using single-photon sources have demonstrated this relationship between individual detection and collective statistical structure. (nist.gov)
If a measurement reliably distinguishes the alternative paths, the interference between them disappears. This relationship connects interference visibility with the information acquired by the measuring apparatus, rather than with a conscious observer. The relevant quantum description concerns both the light and its physical interaction with the apparatus. (nist.gov)
The field description offers a common framework for these observations. A photon is a quantum excitation of a field, not a classical particle with an attached classical wave. Wave propagation and discrete detection are complementary features of that description. (home.web.cern.ch)
Interactions with matter
Matter exchanges energy with radiation through photon absorption and emission. In an atom, radiative transitions connect discrete energy levels, producing characteristic spectral lines. Measurements of photon wavelengths and transition probabilities therefore underpin spectroscopy, which identifies and investigates atomic structures through their radiation. Atomic spectral databases record the associated wavelengths, energy levels, and transition probabilities. (nist.gov)
Photons may also transfer energy to electrons through photoelectric absorption or scattering. At sufficiently high energies, interactions can produce particle–antiparticle pairs. The relative importance of scattering, photoelectric absorption, and pair production depends on photon energy and the material involved; these processes determine how strongly a photon beam is attenuated. (nist.gov)
In stimulated emission, incident radiation prompts an excited system to emit additional radiation into the stimulated mode. This process supplies the amplification mechanism of a laser. In practical lasers, energy is supplied to an active medium, and optical feedback supports amplification into an organized output beam. (nist.gov)
Detection and quantum technologies
Single-photon detectors convert individual light interactions into measurable electrical signals. Technologies include avalanche photodiodes, superconducting nanowire detectors, and transition-edge sensors. Some detectors register an arrival without determining photon number, whereas photon-number-resolving detectors distinguish multiple photons within a pulse. Detection efficiency, noise, dead time, and other response characteristics affect how accurately measurements represent the incoming light. (nist.gov)
Photons also carry quantum information. Two polarization states can encode a qubit, while correlated photon pairs support experiments involving quantum entanglement. Sources, optical components, and sensitive detectors enable applications such as quantum key distribution, quantum teleportation, and tests of quantum correlations. Their implementation depends on controlling photon generation, transmission, and measurement at the single-photon level. (cds.cern.ch)