The paper presents a theoretical analysis of the problem of anomalous reflexion of laser radiation by a plasma due to stimulated scattering by ion-acoustic or electron plasma waves (stimulated Brillouin or Raman scattering). A coherent wave (or well-defined phase) description is used to investigate the case of perpendicular incidence on a plasma layer. The reflexion coefficient as found from the analysis is studied as a function of the power of the laser radiation and of the plasma parameters. The results are compared with those obtained from other investigations. Finally, numerical estimates are given for situations of interest for laser fusion.