  We discuss exotic properties of charged hydrodynamical systems, in the broken
superconducting phase, probed by electromagnetic waves. Motivated by general
arguments from hydrodynamics, we observe that negative refraction, namely the
propagation in opposite directions of the phase velocities and of the energy
flux, is expected for low enough frequencies. We corroborate this general idea
by analyzing a holographic superconductor in the AdS/CFT correspondence, where
the response functions can be explicitly computed. We study the dual
gravitational theory both in the probe and in the backreacted case. We find
that, while in the first case the refractive index is positive at every
frequency, in the second case there is negative refraction at low enough
frequencies. This is in agreement with hydrodynamic considerations.
