Summary

This paper deals with the waves which can be produced inside, as well as in front of and behind a scattering slab, with parallel boundaries, which contains a slightly inhomogeneous medium. The refractive index of the latter is given as a function of the three space variables and the time, without considering possible statistical properties. The theory treating the contributions generated by an obliquely incident wave and due to a successive number of scatterings, is developed with the aid of symbolic expressions depending on partial derivatives with respect to the space-time variables; in particular the twoo-dimensional Laplace operator of the refractive-index distributions in planes parallel to the boundaries is shown to playa dominant role. A resulting representation of the Born approximation (accounting for first-order scatterings only) clearly shows the transition from the mainly scattering behaviour of a thin slab to the predominantly diffracting behaviour of a thick slab. The main waves generated the individual travelling waves (termed acoustic waves) tha