What classical physics couldn't explain
If light were only a wave, even dim light should eventually build up enough energy to free electrons. Instead, red light below a threshold frequency never ejected electrons no matter how bright, while even faint ultraviolet light did so instantly. The electrons' energy depended on frequency, not intensity.
Einstein's photons
In 1905 Einstein proposed that light consists of packets, photons, each with energy E = hf. An electron escapes only if one photon's energy exceeds the metal's work function; the leftover becomes kinetic energy. Brightness just means more photons. Millikan's experiments confirmed it, and Einstein won the 1921 Nobel Prize for this rather than relativity. Wave-particle duality led to quantum mechanics and the uncertainty principle, and the effect powers solar cells and camera sensors.
FAQ
- Is light a particle or a wave?
- Both—interference shows wave behavior; the photoelectric effect shows particle behavior.
- Do solar cells use it?
- Yes, a related internal photoelectric (photovoltaic) effect in semiconductors.
Sources reviewed: 2026-10-26
References
- 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.
- The Nobel Prize in Physics 1921. NobelPrize.org.