Physics
Compton Scattering
Quick fact
Arthur Compton’s 1923 experiment showed that light behaves like a particle when it collides with an electron, earning him the Nobel Prize in Physics in 1927 and forever changing our understanding of electromagnetic radiation.
Why this is interesting
You've seen X-ray images, but did you know that when X-rays hit matter, some photons come out with lower energy? Why does that happen, and what does it tell us about the nature of light?
Read the full explanation
Understanding Compton Scattering
Imagine a photon as a tiny billiard ball of energy and an electron as a stationary ball. When the photon strikes the electron, it transfers some of its energy to the electron, causing the photon to 'bounce off' with less energy. Because a photon's energy is inversely proportional to its wavelength, a loss of energy means an increase in wavelength. This increase is the Compton shift. The exact shift depends on how far the photon is deflected—the scattering angle. The effect is most noticeable for high-energy photons like X-rays or gamma rays, where the wavelength shift is a measurable fraction of their original wavelength.
A deeper explanation
Compton scattering cannot be explained by classical wave theory, which predicts that scattered waves should have the same wavelength as the incident wave. The phenomenon reveals that photons carry momentum (p = h/λ) and that interactions conserve both energy and momentum at the quantum level. The Compton formula (Δλ = h/(me c) (1 - cosθ)) quantifies the wavelength shift, where h is Planck's constant, me the electron mass, c the speed of light, and θ the scattering angle. This result directly confirmed Einstein's light-quantum hypothesis and laid groundwork for quantum electrodynamics. Applications include Compton spectroscopy for studying electron momentum distributions in materials and medical imaging (e.g., Compton cameras in PET scans), as well as radiation shielding design.