Caustics by Refraction of Circles and Lines

Abstract: This short note revisits a classical result that the complete caustic by refraction of a circle is the evolute of a Cartesian oval. We provide additional details to the statement and geometric proof of this fact, as presented in G. Salmon's 1879 book `Higher Plane Curves'. We observe that as the circle tends to a line, this Cartesian oval collapses into an ellipse. Finally, we discuss a computational method to find the complete caustics by refraction, independent of Salmon's proof.