  We experimentally investigate the Bragg reflection of light at
one-dimensionally ordered atomic structures by using cold atoms trapped in a
laser standing wave. By a fine tuning of the periodicity, we reach the regime
of multiple reflection due to the refractive index contrast between layers,
yielding an unprecedented high reflectance efficiency of 80%. This result is
explained by the occurrence of a photonic band gap in such systems, in
accordance with previous predictions.
