A photon is a quantum of the electromagnetic field, meaning that it is the discrete excitation associated with electromagnetic radiation. Photons are the fundamental quanta of light and are involved whenever electromagnetic radiation is emitted, absorbed or otherwise exchanged at the quantum level. A photon has zero rest mass and travels at the invariant speed c in vacuum. Its energy is given by E equals hf, where h is Planck's constant and f is frequency.
Its momentum is related to its energy by p equals E divided by c, giving p equals hf divided by c. A photon can therefore transfer both energy and momentum to matter. Photons do not have electric charge and are not composed of smaller known particles. In quantum field theory, the photon is the gauge boson associated with the electromagnetic interaction. It is therefore both a quantum of electromagnetic radiation and a fundamental mediator of electromagnetic interactions.
Photon energy depends directly on frequency. High frequency photons such as X ray and gamma ray photons have greater individual energies than low frequency radio photons. This does not mean that a low frequency beam necessarily contains less total energy because a beam can contain an enormous number of low energy photons. Total electromagnetic energy depends on the number of photons and their individual energies.
Photon wavelength and frequency are related in vacuum by c equals lambda multiplied by f, while photon energy can also be written E equals hc divided by lambda. These relationships explain why shorter wavelength radiation corresponds to higher photon energy. A photon of visible red light has less energy per photon than a photon of visible violet light because violet light has higher frequency. The same principle extends across the entire electromagnetic spectrum.
The photon concept emerged from attempts to explain experimental results that classical electromagnetic theory could not adequately account for. Max Planck introduced quantisation in his analysis of blackbody radiation, proposing that energy exchange occurred in discrete amounts related to frequency. Albert Einstein extended this idea to electromagnetic radiation when explaining the photoelectric effect. He proposed that light interacts with matter through individual quanta whose energy is hf.
Arthur Compton's scattering experiments later demonstrated that electromagnetic radiation transfers energy and momentum in a manner consistent with photon behaviour. Quantum electrodynamics subsequently developed a more complete theoretical framework. The modern photon is therefore not simply the historical light quantum of early quantum theory. It is a quantum field theoretical entity described by precise mathematical rules governing electromagnetic interactions, emission, absorption, scattering and propagation.
Photons are unusual when compared with ordinary massive particles. They have zero rest mass, zero electric charge and two possible helicity states associated with their spin one nature. Because photons are massless gauge bosons, their behaviour is constrained by electromagnetic gauge symmetry and special relativity. A photon in vacuum always travels at c, whereas massive particles can travel at lower speeds.
A photon does not have a conventional rest frame because there is no inertial frame moving alongside a photon at c. Statements about what a photon experiences in its own frame are therefore not physically equivalent to ordinary statements about an observer's frame. Photons can nevertheless possess well defined energy, momentum, angular momentum, polarisation and other quantum properties. Their interactions with matter can alter their direction, frequency, polarisation and quantum state, while an isolated photon propagating through vacuum does not simply decay because it is moving through space.
Photons can be emitted by many physical processes. An atom can emit a photon when an electron transitions between quantised energy states, with the photon frequency determined by the energy difference. Molecules can emit photons through electronic, vibrational or rotational transitions. Thermal radiation arises from microscopic processes in matter and contains a distribution of photon energies. Accelerated charged particles can generate electromagnetic radiation, including synchrotron radiation and bremsstrahlung. Particle interactions can also produce photons.
Lasers use stimulated emission to generate radiation with distinctive coherence and spectral properties. Semiconductor light emitting diodes produce photons through electronic transitions associated with charge carrier recombination. Photons can subsequently be absorbed by atoms, molecules, electrons or solids, transferring their energy and momentum to the interacting system. These processes are central to spectroscopy, imaging, communications and many forms of optical technology.
Photon detection is fundamentally an interaction between the electromagnetic field and matter. A photodetector does not normally observe an abstract photon directly without an interaction. Instead, the photon is absorbed or otherwise interacts with a material system, producing a measurable change such as an electrical signal. Single photon detectors can register individual detection events. At sufficiently low intensities, experiments can demonstrate that photons arrive as discrete events even though their probability amplitudes can interfere like waves.
This is a central feature of quantum optics. The statistical distribution of detection events can reveal whether a source behaves like coherent classical radiation, thermal radiation or a nonclassical quantum source. Technologies such as avalanche photodiodes, photomultiplier tubes and superconducting nanowire detectors exploit different physical mechanisms to detect photons with varying efficiencies, timing resolutions and wavelength sensitivities.
Photons also possess angular momentum and polarisation, which are important in quantum optics and applications. Circularly polarised photons carry angular momentum associated with helicity, while linearly polarised light represents a particular superposition of polarisation states. Photon polarisation can be manipulated using optical components such as polarisers, wave plates and birefringent materials. Pairs of photons can also be produced in correlated or entangled quantum states, allowing experiments that test fundamental aspects of quantum mechanics and enabling technologies in quantum information science.
Photon statistics are relevant to secure quantum communication, quantum imaging and precision measurement. In conventional optical communication, enormous numbers of photons may be transmitted through fibre, while advanced systems can operate closer to the quantum limit where individual photon statistics and detector noise become important.
A photon should not be pictured as a tiny classical sphere travelling through space along a perfectly defined trajectory. Quantum theory assigns photons quantum states and probability amplitudes, and the outcomes of measurements are probabilistic according to the state. The photon concept is most accurate when describing quantised electromagnetic interactions rather than attempting to impose classical particle properties such as a fixed rest frame or simultaneously definite position and momentum.
Photons can behave in ways that produce interference and diffraction, while their detection occurs in discrete events. This combination is part of quantum theory rather than evidence that photons switch between two unrelated physical forms. The photon is therefore best understood as the quantum of the electromagnetic field, with energy, momentum and angular momentum determined by its quantum state and frequency, and with interactions governed by quantum electrodynamics.
