English: Diffraction theory is concerned with the difference between the actual behavior of time periodic waves and the behavior predicted by geometrical optics. The actual behavior is determined by the solution of a boundary value problem for an appropriate partial differential equation. The geometrical optics prediction is based upon the hypothesis the waves travel along certain straight or curved lines called rays.The relation between these two differential descriptions is that the wave motion determined by geometrical optics is the asymptotic form of the solution of the boundary value problem, as the frequency or the wave number becomes large.

The wave motion predicted by geometrical optics possess certain singularities. It becomes infinite on certain surfaces (or in two dimensions, certain curves) which are called CAUSTICS…….A caustic is the limit of a thin layer within which the solution attains a large finite value which becomes infinite as the parameter tends to its limit…… "Boundary layer problems in diffraction theory"

Authors: R. N. Buchal, J. B. Keller

English: image of caustic generated by the sun into a glass jar