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The fields near a caustic created by an edge diffraction process are computed using the equivalent current concept , . These fields are shown to have the property commonly associated with ray optical analysis or the geometrical theory of diffraction, e.g., a phase shift as the ray passes through the caustic. This result was predicted by Kay and Keller  on the basis of solutions for reflected fields from singly curved surfaces. The present effort is directed toward consideration of the caustic created by an edge diffraction process. Particular attention is focused on electromagnetic excitation. The acoustic excitation for the hard boundary condition is outlined in the Appendix. In addition, our present goal is to establish the extent of the caustic region. This is of particular importance when a ray optical solution involves multiply diffracted terms in that the minimum size of the body that can be analyzed may be restricted by the extent of the caustic, i.e., the phase shift used in ray optical analysis may be introduced only if the caustic is contained on the surface being studied.