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).
See alsoCatacaustic, Circle Catacaustic, Diacaustic, Envelope, Evolute, Orthotomic, Radiant Point
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ReferencesLawrence, J. D. A Catalog of Special Plane Curves. New York: Dover, p. 60, 1972.Lockwood, E. H. "Caustic Curves." Ch. 24 in A Book of Curves. Cambridge, England: Cambridge University Press, pp. 182-185, 1967.Wells, D. The Penguin Dictionary of Curious and Interesting Geometry. London: Penguin, p. 28, 1991.Yates, R. C. "Caustics." A Handbook on Curves and Their Properties. Ann Arbor, MI: J. W. Edwards, pp. 15-20, 1952.
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Weisstein, Eric W. "Caustic." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Caustic.html