Electromagnetic Properties of Insulators. I

We discuss the response of a perfect insulator to weak external electromagnetic fields of long wavelength from a many-particle point of view. Our method is to treat the Coulomb interaction between all electrons to all orders of perturbation theory and analyze the structure of the corresponding Feynman graphs. As a result of this graphical analysis we are able to show that the response of the many-particle system to longwavelength external fields of arbitrary polarization is completely described by a single frequency-dependent dielectric constant. In the limit of long wavelengths as well as low frequencies, we also include, in terms of magnetic permeability, the magnetic effects of an external field on our system.