Who discovered the increase in X-ray wavelength when scattered by electrons?

Arthur Holly Compton discovered the increase in X-ray wavelength when X-rays are scattered by electrons.

Compton observed the effect in 1922 while studying X-rays scattered by light elements and published his explanation in 1923. He proposed that the incoming X-ray behaves as a photon carrying both energy and momentum. When it collides with an electron, the photon transfers some energy to the recoiling electron. Its energy decreases, so its wavelength increases.

The wavelength change depends on the scattering angle and is described by the Compton formula: Δλ = h/(mₑc)(1 − cos θ). This result could not be adequately explained by treating X-rays solely as classical electromagnetic waves. It became important evidence for the particle nature of light and helped establish wave-particle duality.

The phenomenon is called Compton scattering or the Compton effect. It should not be confused with the photoelectric effect, in which a photon ejects an electron, or with Thomson scattering, the classical low-energy limit in which the photon’s wavelength is essentially unchanged. Compton shared the 1927 Nobel Prize in Physics for the discovery.