On gravitational lensing in the presence of a plasma

We discuss in detail our new results in the field of gravitational lensing in a plasma. We show that gravitational deflection by a point mass in a homogeneous plasma differs from vacuum deflection and depends on the frequency of the photon. This frequency dependence resembles the properties of a refractive prism spectrometer, whose strongest action takes place for very long radio waves. We also derive the photon deflection angle by a point mass in an inhomogeneous plasma. Then we consider a more general case of photon deflection by a mass distribution in an inhomogeneous plasma. Our approach allows us to consider two effects simultaneously: the gravitational deflection in a plasma which differs from the vacuum case (a new effect), and the non-relativistic effect (refraction) connected with the plasma inhomogeneity.