text: In an isotropic and linear medium, this polarization field P is proportional and parallel to the electric field E:
  where χ is the electric susceptibility of the medium. The relation between D and E is thus:
  where
  is the dielectric constant of the medium.  The value 1+χ is called the relative permittivity of the medium, and is related to the refractive index n, for non-magnetic media, by