Compton scattering

Compton scattering emerged because a physicist in St. Louis asked what happens when X-rays collide with electrons—and the answer shattered the remaining resistance to quantum physics.

In 1922, Arthur Holly Compton was conducting experiments at Washington University in St. Louis, directing X-rays at a graphite target and carefully measuring the scattered radiation. What he observed defied classical physics: the scattered X-rays had longer wavelengths than the incident X-rays, and the wavelength shift depended on the scattering angle. Classical wave theory predicted no such shift.

The explanation was revolutionary. Compton showed that X-rays must be particles—photons—that collide with electrons like billiard balls. When a photon strikes an electron, it transfers some of its energy and momentum to the electron, emerging with lower energy and therefore longer wavelength. The mathematics were precise: the wavelength shift followed a simple equation involving Planck's constant, the electron mass, and the scattering angle.

This was not the first suggestion that light behaved as particles. Einstein had proposed the photon concept in 1905 to explain the photoelectric effect. But many physicists resisted, preferring to believe that light was purely a wave phenomenon as Maxwell's equations suggested. Compton's experiment was different—it provided direct, measurable evidence of photon momentum. You could calculate the collision like a physics problem about colliding spheres.

The timing was not coincidental. X-ray technology had advanced enough to produce well-characterized beams. Spectroscopic techniques could precisely measure wavelengths. The theoretical framework—quantum mechanics in its nascent form—was ready to interpret the results. Compton was building on Röntgen's discovery of X-rays in 1895, Planck's quantum hypothesis of 1900, Einstein's photon theory of 1905, and Bohr's atomic model of 1913.

Compton published his results in 1923. The physics community recognized the significance immediately. The 'Compton effect' demonstrated wave-particle duality in a form that could not be explained away. Compton received the Nobel Prize in Physics in 1927, sharing it with C.T.R. Wilson.

The discovery enabled practical applications across physics and medicine. Compton scattering remains fundamental to understanding radiation's interaction with matter. Medical imaging, radiation therapy, astrophysics, and materials science all depend on understanding how photons scatter from electrons—the precise phenomenon Compton revealed in his St. Louis laboratory.