  Frequency-dependent gravitational lens effects are found for trajectories of
electromagnetic rays passing through a distribution of plasma near a massive
object. Ray propagation through plasma adds extra terms to the equations of
motion that depend on the plasma refractive index. For low-frequency rays these
refractive effects can dominate, turning the gravitational lens into a mirror.
While light rays behave like particles with an effective mass given by the
plasma frequency in a medium with constant density, an inhomogeneous plasma
introduces more complicated behavior even for the spherically symmetric case.
As a physical example, the pulse profile of a compact object sheathed in a
dense plasma is examined, which introduces dramatic frequency-dependent shifts
from the behavior in vacuum.
