> Impedance is analogous to index of refraction That's not just an analogy; impedance is index of refraction, with the same body of modern theory, except that

> Impedance is analogous to index of refraction That's not just an analogy; impedance is index of refraction, with the same body of modern theory, except that "impedance" derives from the history of analysis of low frequency ("radio") waves whereas "index of refraction" derives from the history of analysis of high frequency ("optical") waves.