Einstein's photoelectric effect: KE = hν − φ — threshold frequency and work function
Einstein (1905, Nobel 1921): light consists of quanta (photons) each with energy E=hν. An electron bound with work function φ is ejected only if hν≥φ, with KE_max = hν−φ (independent of intensity). Intensity controls only the photocurrent (number of electrons), not their energy — direct evidence for quantization. Millikan measured h/e from the slope of KE vs ν (1916), confirming Einstein's formula to 0.5%.