Compton scattering
/ COMP-ton /
When a beam of light bounces off a free electron, you might expect the reflected light to keep its original color, the way a ball keeps its size after bouncing. Instead, X-rays scattering off electrons come back with a slightly longer wavelength — a slightly redder color — and the shift depends on the angle. This is Compton scattering, discovered by Arthur Compton in 1923, and it was decisive proof that light really does come in particle-like quanta.
The explanation is a simple billiard-ball collision, but between a photon and an electron. A photon carries both energy and momentum, and when it strikes a stationary electron it knocks the electron forward, handing over some of its energy and momentum. Having given energy away, the photon emerges with less energy, which for light means a longer wavelength. Apply ordinary conservation of energy and momentum to the photon-electron collision and you predict the exact wavelength change with angle — a prediction that matches experiment beautifully and makes no sense if light were only a wave.
In quantum electrodynamics, Compton scattering is one of the textbook processes, built from QED's basic vertices: an electron absorbs the incoming photon and re-emits an outgoing one. Computing its probability — the Compton cross-section — was an early triumph of the theory. The same physics matters far beyond the lab: Compton scattering governs how X-rays and gamma rays penetrate matter, underpins medical imaging and radiation shielding, and shapes how light from the hot early universe interacted with electrons.
Fire an X-ray photon at a loose electron and let it bounce off sideways. The deflected X-ray comes back redder than it went in, and the electron flies off carrying the energy the photon lost — a literal billiard-ball collision proving light has momentum.
Light bouncing off an electron loses energy — proof the photon is a particle with momentum.
Compton scattering treats the electron as essentially free; light bouncing off tightly bound electrons in an atom barely changes wavelength, a closely related process called Rayleigh scattering. (Thomson scattering, by contrast, is the low-energy limit of Compton scattering off a free electron.)