Glossary / History of physics
Light quantum
The particle-like packet of energy that light behaves as. Now called the photon.
In his 1905 paper "On a Heuristic Point of View Concerning the Production and Transformation of Light", Einstein argued that the photoelectric effect and other phenomena make sense if light's energy is localised in packets rather than spread continuously through space[1].
Interference experiments had established that light is a wave, so the idea was resisted for years. The decisive evidence was the Compton effect in 1923, after which the name "photon" spread. Einstein's 1921 Nobel Prize was for the law of the photoelectric effect, not for relativity.
Related terms
- Photoelectric effectThe ejection of electrons from a metal when light shines on it. The energy of each electron depends on the light's frequency (colour), not its intensity.
- Planck constantThe fundamental constant, written h, that sets the step size of energy for light and matter. Frequency times h is the smallest unit of energy for that light.
- Compton effectThe increase in wavelength of X-rays scattered by electrons. It showed light colliding with electrons as a particle carrying momentum.