  We prove that the electromagnetic fields in dielectric media whose
susceptibility follows a fractional power-law dependence in a wide frequency
range can be described by differential equations with time derivatives of
noninteger order. We obtain fractional integro-differential equations for
electromagnetic waves in a dielectric. The electromagnetic fields in
dielectrics demonstrate a fractional power-law relaxation. The fractional
integro-differential equations for electromagnetic waves are common to a wide
class of dielectric media regardless of the type of physical structure, the
chemical composition, or the nature of the polarizing species (dipoles,
electrons, or ions).
