The Green’s function in some potential scattering problems of spherical symmetry is written as an integral of the product of the Green’s functionsG a corresponding to the angular variables andG r corresponding to the radial variables. The integration is carried along a suitable lineC in the plane of complex angular momentum. If C surrounds the spectrum ofG a the usual partial waves expansion is obtained, ifC surrounds the spectrum ofG r a different expansion results which is the most suitable for describing collisions involving small wave lenghts. In particular it is shown in Section 3 how the Green’s function which characterizes a wave propagation phenomenon approaches the classical limit, leading to the ray propagation of ordinary mechanics.