  In [5] we investigated the response of vacuum energy to a gravitational field
by considering a Casimir apparatus in a weak gravitational field. Our approach
was based on a conjecture involving the interpretation of spacetime as a
refractive medium and its effect on vacuum energy composed of virtual massless
scalar particles. There it was shown how the case of virtual photons as the
constituents of vacuum could be inferred from that of the massless scalar
field. Here we explicitly show how the same conjecture applies to the
electromagnetic vacuum composed of virtual photons. Specifically we show that
the boundary conditions imposed on the components of the vector field,
decomposed into two scalar fields, result in the same frequency shift for
photons. Using the same decomposition and employing our conjecture, we also
calculate the electromagnetic energy density for the Casimir apparatus in a
weak gravitational field.
