- The axion–photon system in an external magnetic field, when for example considered with the geometry of the experiments exploring axion–photon mixing, displays a continuous axion–photon duality symmetry in the limit the axion mass is neglected. The conservation law that follows from this symmetry is obtained. The magnetic field interaction is seen to be equivalent to first order to the interaction of a complex charged field with an external electric potential, where this fictitious "electric potential" is proportional to the external magnetic field. This allows one to solve for the scattering amplitudes using already known scalar QED results. It is argued that in more generic conditions (not just related to these experiments) axion–photon condensation could be obtained for high magnetic fields. Finally an exact constraint originating from the current conservation on the amplitudes of reflected and transmitted waves is obtained.