All kind of information from distant celestial bodies comes to us in the form
of electromagnetic radiation. In most cases the propagation of this radiation
can be described, as a reasonable approximation, in terms of rays. This is
true not only in the optical range but also in the radio range of the
electromagnetic spectrum. For this reason the laws of ray optics are of fundamental
importance for astronomy, astrophysics, and cosmology.
According to general relativity, light rays are the light-like geodesies of a
Lorentzian metric by which the spacetime geometry is described. This,
however, is true only as long as the light rays are freely propagating under the
only influence of the gravitational field which is coded in the spacetime
geometry. If a light ray is influenced, in addition, by an optical medium (e.g.,
by a plasma), then it will not follow a light-like geodesic of the spacetime
metric. It is true that for electromagnetic radiation traveling through the
universe usually the influence of a medium on the path of the ray and on
the frequency is small. However, there are several cases in which this
influence is very well measurable, in particular in the radio range. For example,
the deflection of radio rays in the gravitational field of the Sun is
considerably influenced by the Solar corona. Moreover, current and planned Doppler
experiments with microwaves in the Solar system reach an accuracy in the
frequency of ?? /? = 10-15 which makes it necessary to take the influence
of the interplanetary medium into account. Finally, even in cases where the
quantitative influence of the medium is negligibly small it is interesting to
ask in which way the qualitative aspects of the theory are influenced by the
medium. The latter remark applies, in particular, to the intriguing theory of
gravitational lensing.