Compton scattering

PhysicsMeasurement & UncertaintyCommunity

An X-ray photon hits a free electron at rest. The photon scatters at an angle θ and the electron recoils. Students change the angle and the incoming wavelength, then watch the wavelength shift Δλ = (h/mₑc)(1 − cos θ) and the energy given to the electron.

Lesson: When an X-ray photon collides with a free electron, it behaves like a particle with energy E = hc/λ and momentum p = h/λ. Conservation of energy and momentum shows that the scattered photon has a longer wavelength: Δλ = (h/mₑc)(1 − cos θ), where h/mₑc = 2.43 pm is the Compton wavelength of the electron. The shift is 0 at θ = 0°, 2.43 pm at 90° and 4.86 pm at 180°. It does not depend on the incoming wavelength, so it is only noticeable for short wavelengths such as X-rays and gamma rays. The energy lost by the photon becomes the kinetic energy of the electron. Compton's 1923 experiment was strong evidence that light carries momentum in packets, as the particle picture predicts.

Created by a teacher with Simulic AI and reviewed by the Simulic team.