Reflection high-energy electron diffraction, which is a very widely used monitoring technique of molecular-beam-epitaxial growth processes, has still some unexplained features. An interesting example of these, the so-called t(3/2)/T phenomenon, is investigated in this work. The first period of the intensity oscillations of reflection high-energy electron diffraction shows a singular behaviour. An interpretation for the initial change of the phase and of the period duration dependence on the incident angle of the electron beam using the notion of surface coherence length is given here. This particular phenomenon is satisfactorily explained in the case of a GaAs (001) surface.