On structure of the polarization operator in a magnetic field

The polarization operator is investigated at arbitrary photon energy in a constant and homogeneous magnetic field for the strength H less than the Schwinger critical value. The effective mass of a real photon with a preset polarization is considered in the quantum energy region as well as in the quasiclassical one. Obtained in the quantum region expressions include the singular terms at the creation threshold of electron and positron on Landau levels.