E.2 · IB Physics
Compton scattering
Fire a high-energy photon at a free electron at rest and choose the scattering angle. See how the wavelength shift, the recoil direction of the electron and the share of energy change.
Drag above the axis on the diagram to set θ. Drawn wavelengths are exaggerated; their ratio λ′/λ is to scale (capped for very short λ).
Δλ = λ′ − λ = (h / mₑc)(1 − cos θ)
h / mₑc = 2.43 × 10⁻¹² m
E = hc / λ ; Eₖ = E − E′
Δλ
λ′
E (incident)
E′ (scattered)
Eₖ electron
Energy to electron
Electron angle φ
Try this
- Set θ = 90° and change λ from 1 pm to 100 pm. Does Δλ change? Compare it with h/mₑc.
- At θ = 180°, compare the fraction of energy given to the electron for λ = 2 pm and λ = 100 pm.
- Sweep θ from 0° to 180° and watch φ: why does the electron never move backwards?
Exam tip: the shift depends only on θ, so add Δλ to λ first and then use E = hc/λ′; never subtract a shift directly from the photon energy.