  Accretion onto black holes often proceeds via an accretion disk or a
temporary disk-like pattern. Variability features observed in light curves as
well as theoretical models of accretion flows suggest that accretion disks tend
to be inhomogeneous -- variety of substructures (clumps) emerge within the
flow. Rapid orbital motion of individual clumps then modulates the observed
signal in X-rays. Furthermore, changes of spectral lines and polarization
properties of the observed signal (or the absence of changes) constrain the
models and reveal information about general relativity (GR) effects. In this
write-up we summarize the basic equations that have been employed to study
light propagation near black holes and to derive the radiation signal that can
be expected at a detector within the framework of geometrical optics
approximation.
