Right now the mirror operation only supports mirroring through (across) an infinite plane - which is fine as far as it goes, but when I'm working on optics I'd find it a lot handier to mirror BY a plane. As it is right now, for each inbound ray I want to trace I have to create an infinite plane at each reflective surface, located at the point of intersection of the ray with the surface and whose normal is the projection of the ray onto the surface.
So for example (see attached screen shot and .tcp file, TCPro v10 build 1359) - a simple case with two perpendicular mirrors - I had to draw six (dashed blue) projections and create six infinite planes (not shown) to be able to trace three (green, orange, purple) rays. This would be MUCH easier if I could reflect off planes coincident with the mirror surfaces, which would take only two planes and wouldn't require manually creating the projection lines.
As an aside, it seems I can only select an infinite plane object (e.g. to option-drag a copy to another location) when it's reasonably close to face-on in the view I currently have - except when I'm mirroring, in which case I can select it even when it's edge-on. I spend a lot of time flipping the view back and forth between being ale to select it and then seeing where I'm moving it to.