  We study the phenomenon of additional light waves (ALWs), observed in crystal
optics: two or more electromagnetic waves with the same polarization, but
different refractive index, propagate simultaneously in a isotropic medium. We
show that ALWs are common in relativistic hydrodynamics, and in particular in
strongly coupled systems that admit a dual gravitational description, where the
ALWs are dual to quasi normal modes in the AdS gravity. We study both the
transverse and the longitudinal light wave propagation. In the longitudinal
channel we find a transition between regimes with different number of excitonic
resonances which resembles the transition to standard optics observed in
crystals.
