Photoelectric Effect
A quantum theory of light explaining why electrons are ejected from metals only above a threshold frequency, earning the 1921 Nobel Prize in Physics.
The photoelectric effect — the emission of electrons from a metal surface when illuminated by light — had been known since the experiments of Heinrich Hertz in 1887, but its features resisted any explanation within classical wave optics. Classical theory predicted that the kinetic energy of emitted electrons should increase with the intensity of the incident light; instead, experiments showed that the maximum kinetic energy depends only on the frequency of the light and not at all on its intensity. Below a certain threshold frequency, no electrons are emitted regardless of how bright the source. The electron emission rate, not the energy, grows with intensity.
The resolution published in 1905 in the paper Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt (“On a Heuristic Point of View Concerning the Production and Transformation of Light”) was audacious: light itself is quantized. Building on Planck’s earlier work on black-body radiation, the paper proposed that light is composed of discrete energy quanta (later named photons by Gilbert Lewis) each carrying energy E = hν, where h is Planck’s constant and ν is the frequency. An electron can absorb exactly one quantum; if hν exceeds the work function of the metal, the electron escapes with the residual kinetic energy hν − φ. This linear relation between photon frequency and maximum electron kinetic energy was subsequently verified with great precision by Robert Millikan, who, despite initially hoping to disprove the quantum hypothesis, found perfect agreement with the prediction.
It was for this explanation — not for special or general relativity — that the Nobel Prize in Physics for 1921 was awarded. The work established the wave–particle duality of light and opened the door to quantum mechanics. The concept of the photon remains central to quantum electrodynamics, and the photoelectric effect itself is the operating principle behind photovoltaic cells, photomultiplier tubes, and X-ray photoelectron spectroscopy.
