SUMMARY. “Treating the photon as a particle with momentum that can be transferred to an electron leads to a theoretical Compton shift that agrees with the wavelength shift measured in the experiment. This provides evidence that radiation consists of photons.
https://opentextbc.ca/universityphysicsv3openstax/chapter/the-compton-effect/#:~:text=The%20Compton%20effect%20is%20the,wavelength%20of%20the%20incident%20radiation