Which 1923 discovery convinced many physicists that light was composed of photons?

The 1923 discovery that convinced many physicists light was composed of photons was Compton scattering.

Arthur Holly Compton found that X-rays scattered by electrons emerged with a longer wavelength than the incoming radiation. The amount of wavelength change depended on the scattering angle, matching a collision model in which an X-ray photon transfers energy and momentum to a recoiling electron.

Classical wave theory could not adequately explain this wavelength shift, especially at low intensities. Compton’s result therefore supplied strong evidence that light quanta behaved as particle-like objects, complementing Einstein’s 1905 light-quantum explanation of the photoelectric effect.

The distinction between the two phenomena is a common source of confusion: the photoelectric effect helped establish quantized energy exchange, while Compton scattering directly demonstrated photon momentum through X-ray collisions. Bohr and his colleagues still resisted a fully quantized account for a time, but later experiments supported energy and momentum conservation in individual scattering events.