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

The incident photon energy isSubtracting the work function gives $K_{\max }=0.768952\mathrm{eV}=1.2320\times 10^{-19}\mathrm{J}$. This is much less than the electron rest energy, so nonrelativistic kinetic energy is adequate:The magnitude of the retarding stopping potential satisfies $e{V}_s=K_{\max }$, soThe collector must be negative relative to the emitting surface to retard the electrons; the quoted number is the magnitude. Contact-potential corrections are neglected.