  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.
