A laser, a grid, and a detector that has to catch the photon a hundred percent of the time. Drag an optical element out of the tray, drop it on the board, click it to rotate, then start the laser and watch where the light actually goes, which for the first few levels is not where you thought.
Then it stops being about reflections. The photon takes every path at once, the paths interfere, and getting to a hundred percent means arranging things so the wrong routes cancel each other out. There are over fifty levels, an encyclopedia for when a piece stops making sense, and a sandbox for when you want to break something on purpose. It is a real puzzle game that happens to be teaching you quantum optics, and it never once stops to tell you that is what it is doing.