Gravitational Lensing in presence of Plasma: Strong Lens Systems, Black
  Hole Lensing and Shadow
  In this article, we present an overview of the new developments in problems
of the plasma influence on the effects of gravitational lensing, complemented
by pieces of new material and relevant discussions. Deflection of light in the
presence of gravity and plasma is determined by a complex combination of
various physical phenomena: gravity, dispersion, refraction. In particular, the
gravitational deflection itself, in a homogeneous plasma without refraction,
differs from the vacuum one and depends on the frequency of the photon. In an
inhomogeneous plasma, chromatic refraction also takes place. We describe
chromatic effects in strong lens systems including a shift of angular position
of image and a change in magnification. We also investigate high-order images
that arise when lensing on a black hole surrounded by homogeneous plasma. The
recent results of analytical studies of the effect of plasma on the shadow of
the Schwarzschild and Kerr black holes are presented.
