ABSTRACT
All materials are to some extent dispersive. If a field applied to a material undergoes a sufficiently rapid change, there is a time lag in the response of the polarization or magnetization of the atoms. It has been found that such materials have constitutive relations involving products in the frequency domain, and that the frequency-domain constitutive parameters are complex, frequency-dependent quantities. The real parts of the constitutive parameters are even functions of frequency, and the imaginary parts are odd functions of frequency. At low frequencies the permittivity reduces to the electrostatic permittivity. The chapter demonstrates that causality leads to explicit relationships between the real and imaginary parts of the frequency-domain constitutive parameters. The Kronig-Kramers relations imply that if the constitutive parameters of a material are frequency-dependent, they must have both real and imaginary parts; such a material, if isotropic, must be lossy.
