Past exam of the short options paper of the physics Prelims of the University of Oxford / 2021 / S03 / 2 / d

An electron accelerated from negligible initial kinetic energy through $V$ gains $eV$. The largest possible single photon energy in an X-ray tube is this entire kinetic energy, so the Duane-Hunt law givesThe massive anode can absorb the required recoil momentum with negligible recoil energy; most actual events produce lower-energy bremsstrahlung photons. The acceleration energy $eV$ is valid without approximating the electron speed nonrelativistically.