A light source at infinite distance from the circle produces some parallel light rays (in yellow). The rays are reflected by the circle. If the rays have more than one intersection point with the circle, they are reflected at both points. The reflected rays and their continuations are shown in light blue.
Definition: The envelope of the reflected rays by the circle is called the catacaustic of the circle (also - caustic).

Drag the first slider - "numberRays" to change the number of light rays.

Drag the second slider - to change the radius of the circle.

To experiment with the equation of the catacaustic — The Nephroid, click on the check box and change the coefficient a to fit the graph of the Nephroid onto the catacaustic of the circle.