Compton scattering

A photon interacts with a free electron, and the photon that emerges has lower energy and longer wavelength than the one that arrived (15.6.A.1). The photon survives the interaction rather than being absorbed, which is the contrast with the photoelectric effect. 15.6.A.2.i treats the photon as a particle and solves the event with conservation of energy and conservation of momentum, exactly as for any collision.

The sheet prints the shift.

Here $\theta$ is the angle the photon's direction turned through. Read what is absent: the incoming wavelength. The change in wavelength at a given angle is the same for every incoming photon. A photon that carries straight on, $\theta = 0$, is unshifted; one that comes straight back, $\theta = 180$ degrees, takes the largest shift, $2h/(m_e c)$.

The sheet's constants give $h/(m_e c) = 2.43 \times 10^{-12}$ m, so the absolute shift is tiny. The CED never evaluates that length and never uses the phrase Compton wavelength, so treat it as arithmetic you do rather than a value you quote.

Its role in the course is evidential. 15.6.A.2 lists Compton scattering as evidence that light is a collection of discrete, quantized energy packets, alongside the photoelectric effect. The Topic 15.6 boundary statement widens the treatment rather than limiting it, promising momentum conservation in two dimensions both quantitatively and qualitatively.