  We analytically derive a rigorous expression for the relative impedance ratio
between two photonic structures based on their electromagnetic interaction. Our
approach generalizes the physical meaning of the impedance to a measure for the
reciprocity-based overlap of eigenmodes. The consistence with known cases in
the radiofrequency and optical domain is shown. The analysis reveals where the
applicability of simple circuit parameters ends and how the impedance can be
interpreted beyond this point. We illustrate our approach by successfully
describing a Bragg reflector that terminates an insulator-metal-insulator
plasmonic waveguide in the near-infrared by our mpedance concept.
