Ch 1.4 Scattering of a photon from a free electron, resulting in a wavelength shift $\Delta\lambda = (\lambda_C)(1 - \cos\theta)$ where $\lambda_C = h/(m_e c) = 0.00243$ nm. Demonstrates that photons carry momentum $p = h/\lambda$. *See also:* Photon, Compton wavelength.