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Accretion onto black holes often proceeds via an accretion disk or atemporary disk-like pattern. Variability features observed in light curves aswell as theoretical models of accretion flows suggest that accretion disks tendto be inhomogeneous -- variety of substructures (clumps) emerge within theflow. Rapid orbital motion of individual clumps then modulates the observedsignal in X-rays. Furthermore, changes of spectral lines and polarizationproperties of the observed signal (or the absence of changes) constrain themodels and reveal information about general relativity (GR) effects. In thiswrite-up we summarize the basic equations that have been employed to studylight propagation near black holes and to derive the radiation signal that canbe expected at a detector within the framework of geometrical opticsapproximation.

Light rays and wave fronts in strong gravity