Past exam of the short options paper of the physics Prelims of the University of Oxford / 2021 / S03 / 3 / d

The emitted photon energy is the difference of the two energy levels:Using $E_{\gamma }=hc/\lambda$ givesNo numerical mass or radius is supplied, so this is the determinate result. It is the wavelength conditional on the transition occurring. An isolated particle on a circle with no coupling would remain in its energy eigenstate; a charged particle coupled to electromagnetic radiation can emit subject to its transition selection rules. The $3\to 2$ change has $\Delta n=-1$, compatible with an electric-dipole angular factor for a charged ring particle.