What is the photoelectric effect?

When ultraviolet light falls on a clean zinc plate, electrons called photoelectrons are emitted. Alkali metals such as sodium, potassium and caesium emit electrons even with visible light. In an evacuated tube, the emitted electrons travel from the emitter plate to a collector plate and form a photoelectric current. The negative collector potential that just stops the most energetic electrons is the stopping potential V₀, with eV₀ = Kmax.

Experiments show four facts. Photocurrent is proportional to the intensity of light. The stopping potential, and hence the maximum kinetic energy, depends on frequency but not on intensity. Below the threshold frequency ν₀ no electrons are emitted, however intense the light. Emission is practically instantaneous, even with very dim light. The wave theory of light cannot explain the last three facts.

Einstein proposed that light consists of photons, each with energy E = hν, where h = 6.63 × 10⁻³⁴ J s is Planck's constant. One photon gives all its energy to one electron. Part of it, the work function φ₀, frees the electron from the metal; the rest becomes kinetic energy: Kmax = hν − φ₀, or eV₀ = hν − φ₀. If hν < φ₀, no electron escapes. Brighter light means more photons, so more electrons, but not faster ones.