Einsteinpedia

Theories · 1905

The Photoelectric Effect and Light Quanta

In March 1905 Einstein proposed that light itself comes in discrete packets of energy. It was the idea he called revolutionary, and the one named in his Nobel Prize.

Kmax⁡=hν−ϕ

The Puzzle

Shine light on a metal and electrons can be knocked out of it. Heinrich Hertz noticed the effect in 1887, and by 1902 Philipp Lenard had measured something the wave theory of light could not explain: making the light brighter released more electrons, but did not make any of them faster. Only changing the color of the light did that. Below a certain frequency, no electrons came out at all, however intense the beam.

If light were purely a wave, a brighter wave should shake electrons harder. It did not.

Einstein's Idea

In a paper received by the Annalen der Physik on 18 March 1905, "On a Heuristic Point of View about the Creation and Conversion of Light," Einstein suggested that the energy of light is not spread smoothly through space. It travels in separate quanta, each carrying an energy set by the frequency ν of the light:

E=hν

One quantum gives its energy to one electron. Some of that energy, the work function ϕ, is spent escaping the metal, and the rest becomes the electron's motion. That gives the law at the top of this page: the maximum kinetic energy rises in a straight line with frequency, and intensity only changes how many electrons are released.

The photoelectric effect was one application among several. The heart of the paper is an argument about the entropy of radiation, showing that high-frequency light behaves statistically like a gas of independent particles.

How It Was Confirmed

Robert Millikan spent a decade testing the prediction, expecting to disprove it. By 1916 his measurements matched Einstein's straight line precisely, although Millikan still found the idea of light particles hard to accept. Arthur Compton's experiments of 1922 and 1923, in which X-rays bounced off electrons like billiard balls, persuaded most remaining doubters. The word "photon" caught on after the chemist Gilbert Lewis proposed it in 1926 (it had been used earlier in other senses); Einstein had written "light quantum."

What People Get Wrong

  • Einstein did not discover the photoelectric effect. He explained it.
  • Max Planck, in 1900, had quantized the energy exchanged by matter, not light itself. Treating light as quanta was Einstein's step, and Planck resisted it for years.
  • The 1921 Nobel Prize was awarded "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect" and not for relativity.

Where It Stands

Light quanta are a foundation of quantum physics. Every device that turns light into an electrical signal, from a camera sensor to a photomultiplier tube, works because light arrives in packets.

Sources

  1. Nobel Prize in Physics 1921 — facts
  2. American Physical Society — Einstein and the photoelectric effect
  3. The Collected Papers of Albert Einstein, Volume 2 (English translation, archived copy)
  4. Britannica — Photoelectric effect
  5. American Physical Society, Physics — Quantum milestones, 1916: Millikan's measurement of Planck's constant
  6. American Institute of Physics — Einstein chronology for 1905
  7. American Institute of Physics — Einstein exhibit: the great works
  8. Kragh — Photon: new light on an old name
  9. Nobel Prize — Arthur Compton, facts