= Solution
<Compton scattering> transfers <energy> and <momentum> from a photon to an electron. For an initially stationary, approximately free <electron>, a photon scattered through a nonzero angle generally has lower <frequency> and longer <wavelength>; the recoiling electron carries the difference. The effect demonstrates the photon <momentum> $p_\gamma=h\nu/c=h/\lambda$, beyond a purely elastic classical scattering description.
The middle panel of the figure in <past-exam-of-the-short-options-paper-of-the-physics-prelims-of-the-university-of-oxford/2019/s3/1/b/solution> shows the incident photon along $+x$, the outgoing photon at angle $\theta$ above that direction, and the recoiling <electron> at deflection angle $\phi$ below it. Each arrow labels its particle and <momentum>; the initial electron is at rest at the interaction point. The scattered photon and electron have opposite transverse momentum components, while their forward components sum to the incident photon <momentum>.
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