Physics

In Compton scattering, the wavelength shift depends on how much the photon's direction changes. The key insight is that when a photon scatters at angle $\theta = 0$, it continues straight ahead with no deflection—meaning it hasn't really "collided" with the electron in a way that transfers momentum. Since $\Delta\lambda$ is proportional to $1 - \cos\theta$, a forward scattering angle makes this factor vanish, leaving no wavelength change at all.

In Compton scattering, the wavelength shift depends on how much the photon's direction changes. The key insight is that when a photon scatters at angle $\theta = 0$, it continues straight ahead with no deflection—meaning it hasn't really "collided" with the electron in a way that transfers momentum. Since $\Delta\lambda$ is proportional to $1 - \cos\theta$, a forward scattering angle makes this factor vanish, leaving no wavelength change at all.