A light source at the radiant point S produces some radial light rays (in yellow). The rays are reflected by the circle, with radius r=1. 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.[br][br]Definition: The envelope of the reflected rays by the circle is called the [i]catacaustic[/i] of the circle (also caustic).[br][br][list][br][*]Drag the slider - "d" to change the distance from the center of the circle to the radiant point.[br][*]Drag the slider - "numberRays" to change the number of light rays. [br][*]To see the equation of the catacaustic, click on the check box.[br][/list]