Is light a particle or a wave?
In his 1704 Opticks Isaac Newton described light as a stream of tiny corpuscles, while Christiaan Huygens argued it was a wave. Thomas Young's double-slit experiment of 1801 revealed interference fringes, and in the 1860s James Clerk Maxwell explained light as an electromagnetic wave, so the wave picture seemed to win. But in 1900 Max Planck, trying to explain the spectrum of light from hot bodies, assumed that energy is exchanged only in whole multiples of hf. In 1905 Albert Einstein went further, proposing that light itself consists of energy quanta, which explained the photoelectric effect and won him the 1921 Nobel Prize. Robert Millikan set out to refute the idea, but his precise 1916 measurements confirmed Einstein's equation instead.
A name, and a dual nature
In 1923 Arthur Compton showed that X-rays colliding with electrons exchange momentum like billiard balls and come away with longer wavelengths, proving that light quanta carry momentum as true particles. The chemist Gilbert Lewis coined the word "photon" in 1926, and in 1927 Paul Dirac treated emission and absorption of light with a quantised field, laying the foundation of quantum electrodynamics. Even sent one at a time through a double slit, photons build up an interference pattern. Showing up as particle or wave depending on how it is measured — wave–particle duality — is at the heart of quantum mechanics. Being massless, photons always travel at about 300,000 km/s in a vacuum, the speed that anchors relativity.
Technologies built on photons
When an electron in an atom drops between energy levels it emits a photon of a definite colour, and amplifying such photons in step gives the laser. Robert Hanbury Brown and Richard Twiss's 1956 photon-correlation experiment and Roy Glauber's 1963 quantum theory of optics (Nobel Prize 2005) opened the way to treating light photon by photon. In 1982 Alain Aspect used entangled photon pairs to show violations of Bell's inequalities, sharing the 2022 Nobel Prize with John Clauser and Anton Zeilinger. Today photons carry quantum-encrypted messages and are counted by camera sensors, solar cells and telescopes mapping the cosmic microwave background. The rod cells in our eyes can respond to just a handful of photons.
FAQ
- How can a massless photon have energy and momentum?
- In relativity, energy and momentum can exist without mass. A photon's energy is proportional to its frequency (E = hf), and its momentum is that energy divided by the speed of light.
- Do photons wear out or decay?
- Photons do not decay in a vacuum. They disappear when absorbed by matter, and cosmic expansion can stretch their wavelength and lower their energy.
Sources reviewed: 2026-10-26
References
- Newton, I. (1704). Opticks. London: Sam. Smith & Benj. Walford.
- Planck, M. (1901). Über das Gesetz der Energieverteilung im Normalspectrum. Annalen der Physik, 4(3), 553–563.
- Einstein, A. (1905). Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt. Annalen der Physik, 17, 132–148.
- Millikan, R. A. (1916). A direct photoelectric determination of Planck's "h". Physical Review, 7(3), 355–388.
- Compton, A. H. (1923). A quantum theory of the scattering of X-rays by light elements. Physical Review, 21(5), 483–502.
- Lewis, G. N. (1926). The conservation of photons. Nature, 118(2981), 874–875.
- Dirac, P. A. M. (1927). The quantum theory of the emission and absorption of radiation. Proceedings of the Royal Society A, 114(767), 243–265.
- Hanbury Brown, R., & Twiss, R. Q. (1956). Correlation between photons in two coherent beams of light. Nature, 177(4497), 27–29.
- Glauber, R. J. (1963). Coherent and incoherent states of the radiation field. Physical Review, 131(6), 2766–2788.
- Aspect, A., Grangier, P., & Roger, G. (1982). Experimental realization of Einstein-Podolsky-Rosen-Bohm Gedankenexperiment: A new violation of Bell's inequalities. Physical Review Letters, 49(2), 91–94.