n 1905, Albert Einstein published his ground-breaking paper on the Photoelectric Effect—one of four seminal papers released during his annus mirabilis (miracle year). A key implication of his work was that light, traditionally understood as an electromagnetic wave, also exhibits particle-like behavior. This proposed that light quanta (later named photons) must possess momentum, even though they have zero rest mass.
However, direct experimental confirmation of photon momentum was still missing.
The Compton scattering experiment set out to provide definitive proof that photons carry momentum—a key prediction of the quantum model of light.
When high-energy X-ray photons collide with stationary electrons, they scatter and lose energy, resulting in a longer wavelength after the collision. By applying the conservation of energy and relativistic momentum to these photon-electron collisions, Arthur Compton derived a precise mathematical relationship between the photon's wavelength shift and its scattering angle ($\theta$).
Watch the video to explore how Compton derived his scattering formula and how this experiment provided undeniable proof of the particle nature of light.