In 3-D block media the interlaces are represented by the surfaces of the curvilinear polyhedrons. Such surfaces include the vertices, where several diffracting edges can converge, and therefore, in 3-D block media the diffracting edges are not smooth. The latter limits the range of applicability of the approach described in Chapter VIll. The gist of the limitations becomes clear from the following example.
Figure 71a shows the geometry of the shadow boundary of the reflected/transmitted wave in the case of a broken edge. The edge consists of two semi-infinite parts RA and RB. Point R is not a regular point of the edge, because the tangent to the edge cannot be determined as a single value at R. The shadow boundary consists of two parts RAT and RBT.