  In the paper we calculate the frequency shift induced on a photon by the
interaction with a low density electronic plasma. The technique is the standard
perturbation theory of quantum electrodynamics, taking into account the many
body character of the plasma. The shift in non relativistic approximation is
shown to be blue. Besides the quantum shift also the known classical effects
and the correct temperature dependence are obtained. Finally the limits of the
approximations used are discussed.
