We observe coherently enhanced reflection (Bragg scattering) of a near
resonant laser beam from cesium atoms ordered by a periodic array of
optical potential wells known as an optical lattice. The reflected lig
ht is enhanced by a factor of similar to 10(5) over scattering from a
disordered atomic sample. We measure the magnitude of the reflectivity
and its detuning dependence as functions of the density and the degre
e of localization of the atoms. The change in lattice constant due to
the frequency-dependent effective index of refraction seen by the latt
ice light field is observed.