  In the paper we study propagation of light in general relativity through a
spacetime filled with cold plasma with infinite conductivity. We use the
geometric optics based on Synge's approach. As the main result we provide a
formula for calculation of spectral flux of radiation emitted from a pointlike
source. The formula employs connecting vectors that are obtained by integrating
the ray deviation equation along the reference ray connecting the source and
observation event. As a byproduct we formulate Etherington's reciprocity
theorem with the inclusion of plasma, interrelating angular size distance and
luminosity distance. We also discuss the Liouville theorem and its formulation
in terms of connecting vectors.
