I’m very very sorry for not updating but I’ve got it! Here is the proper workings for beams to work. I haven’t ensured that mirrors don’t overlap, but this causes the thing to largely fall over, so avoid it.
(Move to a mirror to select, click on either side to rotate, ctrl+drag to move.)
What’s next from here? Implementing prisms, which will require implying a medium (and relative refractive index) then hopefully thin lenses should come fairly naturally out of that.
What kind of game will this eventually make? I’m hoping something along the lines of Fantastic contraption but including things that require lasers and other beams of light. I expect to be able to burn things soon!