  We derive the wave equation obeyed by electromagnetic fields in curved
spacetime. We find that there are Riemann and Ricci curvature coupling terms to
the photon polarisation which result in a polarisation dependent deviation of
the photon trajectories from null geodesics. Photons are found to have an
effective mass in an external gravitational field and their velocity in a local
inertial frame is in general less than $c$. The effective photon mass in the
Schwarzschild metric is $m_\gamma = ( 2GM/r^3 )^{1/2}$ and near the horizon it
is larger than the Hawking temperature of the blackhole. Our result implies
that Hawking radiation of photons would not take place. We also conclude that
there is no superluminal photon velocity in higher derivative gravity theories
(arising from QED radiative corrections), as has been claimed in literature. We
show that these erroneous claims are due to the neglect of the Riemann and
Ricci coupling terms which exist in Einstein's gravity.
