This .ggb shows rays from an object at inifinity, entering the lens from the left, coming to focus at the lens's focal distance.[br]Note that the angle of the incoming rays is controllable by dragging D, and "Trace On" is set for all rays, so many rays can effectively be traced at at time.[br]You can clear the stored traces by zooming or panning the view (Shift-mousewheel or shift-drag).