Notes
Descartes confusingly employs both letters and numbers to label points. Thus, for example, the line segment from the point 6 to the point R in the diagram will be denoted by 6R.
We consider only circles or disks centered at the point 1, since the results for arbitrary (nonzero) centers are simply scaled/rotated versions of the resulting Minkowski products.
The envelope of a variable circle whose center lies on the circumference of another circle and whose radius is proportional to the distance of its center from a fixed point is a pair of ovals of Descartes.
The extended complex plane is the set of all finite complex values augmented by a single point \(\infty \) at infinity.
For brevity, we consider problems that are rotationally symmetric about a certain axis; the wavefront surfaces are then completely characterized by planar sections.
The name “caustic” is due to Ehrenfried Walther von Tschirnhaus, a contemporary of Leibniz and Newton, who investigated the use of mirrors to focus sunlight: the Latin and Greek roots of “caustic” mean capable of burning.
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Farouki, R.T. The Cartesian Ovals. Math Intelligencer 44, 343–353 (2022). https://doi.org/10.1007/s00283-021-10149-8
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DOI: https://doi.org/10.1007/s00283-021-10149-8