Compton effect
Definition
The Compton effect is X-rays bouncing off electrons and losing energy — proof that light carries momentum like a particle, not just energy like a wave.
Technical definition
A photon scattering off a stationary electron shifts to longer wavelength by Δλ = (h/mₑc)(1 − cosθ). Arthur Compton measured this in 1923, confirming photons have momentum p = h/λ.
Example
An X-ray photon scattering at 90° gains 2.43 pm of wavelength — tiny, but exactly what Compton measured, matching the particle model of light.
Common confusion
The Compton effect is not the photoelectric effect. In the photoelectric effect the photon is absorbed and an electron is ejected; in Compton scattering the photon survives with less energy, bouncing off like a billiard ball.