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a surface that is an envelope of a system of light rays emitted by a luminous point and passing through an optical system. A caustic surface can also be defined as a surface at each of whose points there is an intersection of two rays diverging at an infinitesimal angle from a luminous point and converging after refraction at the boundaries of the system’s optical mediums. The concentration of light energy takes place on a caustic surface and can be seen well in a hazy medium.
The aberrations of optical systems may be classified on the basisof the symmetry properties of caustic surfaces. The axial symme-try of a caustic surface (see Figure 1) corresponds to sphericalaberration, and symmetry with respect to the meridional plane(for example, the plane of the figure) corresponds to the coma.In nonaberrational optical systems the caustic surface becomesa point—the image of a point source.