  The effect of gravitational tidal forces on photon propagation in curved
spacetime is investigated. It is found that the imaginary part of the local
refractive index Im n(u;w) may be negative as well as positive, corresponding
to a local amplification as well as attenuation of the amplitude of the
renormalized photon field. This is interpreted in terms of the effect of tidal
forces on the virtual e^+e^- cloud surrounding the bare photon field---a
positive/negative Im n(u;w) corresponds to an increased dressing/undressing of
the bare photon. Below threshold decays of the photon to e^+e^- pairs can
occur. Photon undressing in the vicinity of a black hole singularity is
described as an example. These results are shown to be consistent with
unitarity and the optical theorem in curved spacetime, which is derived here
both in a local form and integrated over the photon trajectory.
