  The results on the heat kernel expansion for the electromagnetic field in the
background of dielectric media are briefly reviewed. The common approaches to
the calculation of the heat kernel coefficients are discussed from the
viewpoint of their applicability to the electromagnetic field interacting with
dielectric body of arbitrary form. Using the toy-model of scalar photons we
develop multiple reflection expansion method which seems the most promising one
when the field obeys dielectric-like matching conditions on an arbitrary
interface and show that the heat kernel coefficients are expressible through
geometric invariants of the latter.
