To add something such as a mirror, a lens, or an object to the simulation, click the appropriate button above or below the simulation and then click inside the simulation at the position you'd like to place it.

Click-and-drag anything in the simulation to move it or, in the case of an object, to re-size it.

Clicking on a mirror or lens brings up two points, one on either side of the mirror/lens. These are the focal points. Click-and-drag either focal point left or right to change the focal length of the mirror/lens.

Dragging a focal point through a mirror or lens changes the mirror/lens from concave to convex or vice versa.