  Above-barrier reflection of cold alkali atoms by resonant laser light was
considered analytically within the Gross-Pitaevskii approximation. Correction
for the reflection coefficient because of a weak nonlinearity of the stationary
Schroedinger equation has been derived using multiscale analysis as a form of
perturbation theory. The nonlinearity adds spatial harmonics to linear incident
and reflecting waves. It was shown that the role of nonlinearity increases when
the kinetic energy of an atom is nearly to the height of the potential barrier.
Results are compared to the known numerical derivations for wave functions of
the Gross-Pitaevskii equation with the step potential.
