The gravitational lens as a radiospectrometer

Gravitational lensing is predicted by general relativity and is found in observations. Weak lensing and observational examples of lenses, as well as strong lensing with relativistic rings, are considered. When a gravitating body is surrounded by a plasma, the lensing angle depends on a frequency of the electromagnetic wave due to refraction properties, and the dispersion properties of the light propagation in plasma. We consider here the effects of weak and strong gravitational lensing, observations of gravitational lenses, and spectrometric properties of a gravitational lens in a plasma.