  In an optically active medium, such as a plasma that contains a neutrino
background, the left-handed and right-handed polarization photon modes acquire
different dispersion relations. We study the propagation of photons in such a
medium, which is otherwise isotropic, within the framework of the covariant
collissionless Boltzmann equation incorporating a term that parametrizes the
optical activity. Using the linear response approximation, we obtain the
formulas for the components of the photon polarization tensor, expressed in
terms of integrals over the momentum distribution function of the background
particles. The main result here is the formula for the P- and CP-breaking
component of the photon polarization tensor in terms of the parameter involved
in the new term we consider in the Boltzmann equation to describe the effects
of optical activity. We discuss the results for some particular cases, such as
long-wavelength and non-relativistic limits, for illustrate purposes. We also
discuss the generalizations of the P- and CP-breaking term we included in the
Boltzmann equation. In particular we consider the application to a plasma with
a neutrino background and establish contact with calculations of the photon
self-energy in those systems in the framework of Thermal Field Theory.
