Photoelectric Effect

Light ejecting electrons from metal only above a threshold frequency, explained by Einstein as quanta.

Heinrich Hertz noticed in 1887 that ultraviolet light eased spark formation — the first hint of the effect.
Einstein’s 1905 explanation, E = hf − W, treated light as quanta and won him the 1921 Nobel Prize.
Millikan’s decade of precision measurements confirmed the equation and yielded an accurate Planck constant.