  We address the response of a Fabry-Perot interferometer to a monochromatic
point source. We calculate the anticaustics (that is, the virtual wavefronts of
null path difference) resulting from the successive internal reflections
occurring in the system. They turn to be a family of ellipsoids (or
hyperboloids) of revolution, which allows us to reinterpret the operation of
the Fabry-Perot from a geometrical point of view that facilitates comparison
with other apparently disparate arrangements, such as Young's double slit.
