Boundary effects and the massless limit of the photon

We study the effect of a finite photon mass on the vacuum expectation value of the stress tensor for the electromagnetic field near a plane boundary between free space and a perfectly conducting medium. Nonzero vacuum energy contributions arise which diverge at the boundary surface. We compare and contrast the result with that for a real scalar field. We point out the possible existence of new nonminimal-coupling terms in the vector-field Lagrangian which break gauge invariance in the massless limit. In the case that such terms are included, the boundary effects may be measurable.