A novel variational principle in electrostatics and its consequences

•Electrostatic field lines have features common to light rays in geometrical optics.•A novel variational principle in electrostatics is introduced.•It resembles Fermat's principle in optics.•The principle can be used to deduce the ‘mirror equation’ for conducting sphere image problem.

We propose a novel variational principle in electrostatics and show that one can derive mirror equation in the context of image problem starting from this principle.