Gravitational lensing in the presence of plasmas and strong gravitational fields

The theory of gravitational lensing is well developed for light propagation in vacuum in the approximation of weak deflection. We discuss two ways of extending the studies of usual gravitational lens theory which were recently theoretically developed. One way is to go beyond the weak deflection approximation and consider relativistic images formed by the photons performing one or several revolutions around the central object. Another way is to consider the propagation of light in a plasma instead of vacuum, in the presence of gravity. In this case the lensing angle depends on the plasma distribution and the photon frequency. The gravitational deflection of a light ray even in a homogeneous plasma differs from the vacuum (Einstein) angle and depends on the photon frequency. We also discuss the recent results on the influence of a plasma on positions and magnifications of relativistic images.

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