A light source at infinite distance from the parabola with a vertex at the origin and focus and directrix on a distance 1 from the origin, produces parallel light rays (in yellow). The rays are reflected by the parabola. If the rays have more than one intersection point with the parabola, 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 parabola is called the catacaustic of the parabola (also ).

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

Drag the second slider - " " to change the angle between the rays and the directrix of the parabola.

The catacaustic is a Tschirnhausen Cubic. Use the sliders -, horizontal, vertical and coefficient to fit the Tschirnhausen Cubic onto the catacaustic for different values of .