Definition
A photon is an elementary particle that is the quantum of the electromagnetic field, responsible for electromagnetic radiation such as light. It exhibits properties of both particles and waves and is massless, carrying energy and momentum proportional to its frequency.
Overview
In the framework of quantum electrodynamics (QED), photons are gauge bosons that mediate the electromagnetic force between charged particles. Because they have zero rest mass, photons travel at the universal speed limit, c, approximately 299,792,458 m/s in vacuum. Photons are emitted or absorbed in discrete packets during atomic and molecular transitions, and their interactions underpin phenomena ranging from radio transmission to photosynthesis.
Etymology/Origin
The term “photon” derives from the Greek word φῶς (phōs), meaning “light.” It was coined in 1926 by chemist Gilbert N. Lewis to denote a light quantum, building on Albert Einstein’s 1905 proposal of light quanta to explain the photoelectric effect.
Characteristics
| Property | Description |
|---|---|
| Rest mass | Exactly zero (experimentally constrained to < 10⁻⁵⁴ kg). |
| Spin | Intrinsic angular momentum of 1 ħ (bosonic, obeys Bose–Einstein statistics). |
| Charge | Electrically neutral. |
| Energy (E) | Given by $E = h |
| u = \frac{hc}{\lambda}$, where h is Planck’s constant, ν frequency, c speed of light, and λ wavelength. | |
| Momentum (p) | $p = \frac{E}{c} = \frac{h}{\lambda}$. |
| Polarization | Describes the orientation of the electric field vector; can be linear, circular, or elliptical. |
| Interaction | Couples to charged particles via electromagnetic interaction; can be scattered (e.g., Compton scattering) or absorbed/emitted (e.g., atomic transitions). |
| Lifetime | Indefinite in free space; photons persist as long as they are not absorbed. |
| Wave‑particle duality | Exhibits interference and diffraction (wave behavior) and discrete energy exchange (particle behavior). |
Related Topics
- Quantum electrodynamics (QED) – the relativistic quantum field theory describing photon interactions.
- Electromagnetic spectrum – the range of photon frequencies from radio waves to gamma rays.
- Photoelectric effect – emission of electrons from a material upon photon absorption, explained by photon energy quantization.
- Compton scattering – inelastic scattering of photons by electrons, demonstrating photon momentum.
- Bose–Einstein statistics – statistical distribution governing identical bosons such as photons.
- Laser (Light Amplification by Stimulated Emission of Radiation) – a device that produces coherent photons.
- Blackbody radiation – thermal emission modeled as a distribution of photons across frequencies.
Note: The information presented reflects the consensus of scientific literature up to the knowledge cutoff date of September 2021.