Nonzero-mass photons: from the wave equation to the Schrödinger equation

A boundedness of the volume occupied by an electromagnetic field imparts a nonzero mass to the photon. This mass is associated with a standing component of the field. At the same time, the zero-mass wave equation evolves into an equation of the Schrödinger type for particles of nonzero mass. The expressions for the "longitudinal" and "transverse" photon masses are the same as the corresponding relativistic formulas for particles of nonzero mass.