  With ready access to massive computer clusters we may now study light
propagation in a dense cold atomic gas by means of basically exact numerical
simulations. We report on a direct comparison between traditional optics, that
is, electrodynamics of a polarizable medium, and numerical simulations in an
elementary problem of light propagating through a slab of matter. The standard
optics fails already at quite low atom densities, and the failure becomes
dramatic when the average interatomic separation is reduced to around $k^{-1}$,
where $k$ is the wave number of resonant light. The difference between the two
solutions originates from correlations between the atoms induced by
light-mediated dipole-dipole interactions.
