  In recent years a very exciting and intense activity has been devoted to the
understanding and construction of materials that enjoy exotic optical
properties, such as a negative refractive index. Motivated by these
experimental and theoretical developments, we use the string-inspired idea of
holography to study the electromagnetic response of a certain class of media:
strongly coupled relativistic systems that admit a dual gravitational
description. Our results indicate that this type of media generally have a
negative refractive index. Moreover we observe that a negative refractive index
could be a common feature of relativistic hydrodynamic systems at low
frequencies.
