The TI-92 program cusp() draws 20 light rays coming vertically downward, hitting a curved mirror, and then bouncing off. You can enter this program into the TI-92 by typing it in or if your workstation has the TI-Graph Link cable and software by downloading it from your computer.
In order to use cusp you need to define two functions -- mirror and slope that define the shape of the mirror and that give the slope of the mirror at each point. The following screen shows how this is done for the circular mirror in the browser window.
The screen below shows the cusp drawn by the TI-92.
You will need to change the definitions of mirror and slope to work with the parabolic mirror described in the browser window.