A light source at the radiant point S produces some radial 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 called caustic).

Drag the slider - NumberRays to change the number of light rays.

Drag the slider - d to change the distance from the radiant point to the focus of the parabola.

Drag the slider - to rotate the radiant point around the focus of the parabola (the distance d has to be greater than zero).

The focus of this parabola is at the origin and the directrix is on a distance from the x- Axis. Drag slider a to experiment with the catacaustics of different parabolas.