An extension of the technique of ray tracing is studied in which the variation of the signal intensity along the ray is calculated. This results from the divergence or convergence of neighbouring rays in a ray pencil. It is examined for the special case of plane stratification and applied to an anisotropic collisionless ionosphere. With the approximations of ray theory the signal intensity is infinite where a ray touches a caustic surface, and examples of this are given. The ray is said to be reflected where it is parallel to the strata. Here the signal intensity is bounded and continuous but there are singularities in some of the quantities used to calculate it. Methods of dealing with this are discussed.