  The interaction of electromagnetic fields with a solid is characterized by
several interconnected response functions: the dielectric function
$\varepsilon(\omega)$, index of refraction $N(\omega)$, conductivity
$\sigma(\omega)$, and optical impedance $Z(\omega)$. Here we utilize three
canonical models of dielectric response -- the damped harmonic oscillator,
Debye polarization response, and the Drude model -- to discuss these four
optical response functions. Special emphasis is devoted to the response of a
Drude metal. Our main focus is on electromagnetic wave propagation through a
material. We also discuss the relaxation of charge fluctuations within the
context of the three canonical models of response.
