This theoretical article investigates the acoustic field in the image‐interference and shadow‐zone regions for a model in which the sound velocity decreases with increasing depth from the surface. Near the surface the shadow‐zone boundary is the surface‐grazing ray, while in deeper regions this boundary is a caustic. The field near the caustic is investigated by modified ray theory based on Airy functions. A comparison of mode theory results with those of ray theory indicates that there is a −π/2 jump in the ray theory phase when the ray touches a caustic. There is, however, no such jump when the ray becomes horizontal. The normal‐mode series is absolutely convergent for ranges greater than 1.732 times the sum of the source and receiver depths. When the shadow zone is bounded by a well‐developed caustic, the series converges so slowly as to be impractical except at ranges well into the shadow‐zone region.