  We consider the photon field between an unusual configuration of infinite
parallel plates: a perfectly conducting plate $(\epsilon\to\infty)$ and an
infinitely permeable one $\mu\to\infty)$. After quantizing the vector potential
in the Coulomb gauge, we obtain explicit expressions for the vacuum expectation
values of field operators of the form $<{\hat E}_i{\hat E}_j>_0$ and $<{\hat
B}_i{\hat B}_j>_0$. These field correlators allow us to reobtain the Casimir
effect for this set up and to discuss the light velocity shift caused by the
presence of plates (Scharnhorst effect \cite{Scharnhorst,Barton,BarScharn}) for
both scalar and spinor QED.
