  The physical origins of negative refractive index are derived from a dilute
microscopic model, producing a result that is generalized to the dense
condensed phase limit. In particular, scattering from a thin sheet of electric
and magnetic dipoles driven above resonance is used to form a fundamental
description for negative refraction. Of practical significance, loss and
dispersion are implicit in the microscopic model. While naturally occurring
negative index materials are unavailable, ferromagnetic and ferroelectric
materials provide device design opportunities.
