Design a focussing mirror

Looking for lining-up

Move the control points on the mirror to make all the contributing arrows line up at the detector. Once you've done this, notice the shape of the mirror.

You have engineered a focussing mirror by reasoning about a photon exploring paths. And yes, these mirrors do work even if there is only one photon at a time. They're used in astronomical telescopes, looking at very dim stars(so very few photons each second).