Why plane-waves

One reason to use plane-waves instead of rays is to avoid the asymptotic approximation introduced when using rays. Unlike rays, each plane-wave can be treated independently at each frequency. The use of plane-waves simply ignores the problem of non-coplanar rays. Indeed, the original and the rotated rays described earlier share the same plane-wave. The reflection azimuth is the azimuth of the apparent propagation plane which is directly deduced from the angles of the two plane-waves.