Glossary / History of physics
Photoelectric effect
The 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.
In 1902 Philipp Lenard reported that brighter light ejects more electrons but does not change the energy of each one[1] — a property that the wave picture of light cannot explain.
In 1905 Einstein argued that it makes sense if light is made of packets of energy — light quanta — and predicted how electron energy should depend on frequency. In 1916 Robert Millikan confirmed the relation in precise experiments, measuring Planck's constant along the way[2], though he himself doubted light quanta.
Related terms
- Light quantumThe particle-like packet of energy that light behaves as. Now called the photon.
- 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.