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Compton Scattering
Photon momentum transfer — wave-particle duality in quantum mechanics
Δλ = λ' − λ = (h/mₑc)(1 − cosθ) = λ_c(1 − cosθ)
Compton scattering (1923): a photon scatters off a free electron, transferring some momentum and shifting to a longer wavelength. The shift Δλ = λ_c(1−cosθ) depends only on angle — not wavelength! Here λ_c = h/mₑc = 2.426 pm is the Compton wavelength. This experiment proved photons carry momentum p = h/λ, a pillar of quantum mechanics.