Photonna

Meet Photonna, a photon who knocks electrons out of metal, but only if she brings enough energy on her own.

Shine light on a clean metal and electrons can fly off. Experiments around 1900 found puzzling details: below a threshold frequency, no electrons come out however bright the light; above it, the electrons' maximum energy rises with frequency, not brightness. In 1905 Albert Einstein explained this by treating light as quanta, each with energy hf. One photon gives its energy to one electron. The electron must spend the work function φ to escape, and keeps the rest as kinetic energy: K_max = hf − φ.

Brighter light means more photons, so more electrons, but not faster ones. Robert Millikan's measurements confirmed the straight-line relation and gave a value for h. Einstein's 1921 Nobel Prize cited this work.

Photomultiplier tubes and some light sensors use the effect directly.

Real metals have work functions of a few electronvolts, so visible or ultraviolet light is needed. Surface contamination changes φ, which made early experiments hard.