  We firstly revisit the importance, naturalness and limitations of the
so-called optical metrics for describing the propagation of light rays in the
limit of geometric optics. We then exemplify their flexibility and
nontriviality in some nonlinear material media and in the context of nonlinear
theories of the electromagnetism, both underlain by curved backgrounds, where
optical metrics could be flat and impermeable membranes only to photons could
be conceived, respectively. Finally, we underline and discuss the relevance and
potential applications of our analyses in a broad sense, ranging from material
media to compact astrophysical systems.
