Einstein Explains Photoelectric Effect

Albert Einstein publishes 'On a Heuristic Point of View Concerning the Production and Transformation of Light' in Annalen der Physik, proposing that light consists of discrete quanta (later called photons) with energy E=hν. This paper extends Planck's quantization to radiation itself and explains why the photoelectric effect depends on frequency, not intensity — work for which Einstein receives the 1921 Nobel Prize.