  Gravitational lensing is predicted by general relativity and is found in
observations. 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.
The last effect leads to dependence, even in the uniform plasma, of the lensing
angle on the frequency, what resembles the properties of the refractive prism
spectrometer. The strongest action of this spectrometer is for the frequencies
slightly exceeding the plasma frequency, what corresponds to very long
radiowaves.
