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The curve which is the Envelope of reflected (Catacaustic) or refracted (Diacaustic) rays of a given curve for a light source at a given point (known as the Radiant Point). The caustic is the Evolute of the Orthotomic.
References
Lawrence, J. D. A Catalog of Special Plane Curves. New York: Dover, p. 60, 1972.
Lee, X. ``Caustics.''
http://www.best.com/~xah/SpecialPlaneCurves_dir/Caustics_dir/caustics.html.
Lockwood, E. H. ``Caustic Curves.'' Ch. 24 in A Book of Curves. Cambridge, England: Cambridge University Press,
pp. 182-185, 1967.
Yates, R. C. ``Caustics.'' A Handbook on Curves and Their Properties. Ann Arbor, MI: J. W. Edwards, pp. 15-20, 1952.