Get the Tcl plug-in around here (updated 6/01).
The lines reflect off the spherical mirror. You can drag the mirror around and adjust four other parameters. If you move the mouse on a line, the point of reflection will be labeled with the reflecting angle.
The point is that it is easy to see spherical abberation with this demo. The lines don't all intersect at the same point after reflecting in the spherical mirror.
I wrote this while taking Physics 7B at U.C. Berkeley, fall 1997.