The wave generated by an array of s-wave sources located on a three-dimensional lattice is studied between a pair of parallel lattice planes as a function of its (complex) wave vector. The mean current carried by this 'Ewald wave' is determined. The three-dimensional wave generated by a two-dimensional lattice of s-wave sources is also examined. A very simple exact expression is found for the diffraction of a plane wave by the plane surface of a crystal in which the atoms only have an s-wave phase shift which need be neither small nor real. A numerical calculation is made for a wave normally incident on the cube face of a face-centred cubic crystal and the breakdown of the Bragg condition is studied. It is shown how the diffraction could be calculated if the top few layers of the crystal had a different s-wave phase shift. Only if the crystal has such an impurity surface is it possible to have surface waves.