Which historical experiment involving Compton scattering significantly contributed to the development of quantum mechanics by demonstrating the particle nature of light?

The experiment was Arthur Compton’s 1923 study of X‑ray scattering off electrons, now called the Compton effect. He measured that X‑rays of different wavelengths were shifted after striking a target, and the shift depended on the scattering angle exactly as predicted if the X‑rays behaved like particles carrying momentum \(h/\lambda\). This showed that light could be treated as discrete quanta, a key step toward quantum theory. For example, X‑rays of 0. 1 nm scattering at 90° were found to have a longer wavelength, confirming the photon picture.