The mirror revisited

Choose paths a long way from inbound angle = outbound angle and it seems that you can ignore these paths. But something's happening out here—each photon does explore these paths.

Less means more

Focus on the paths a long way away from inbound angle = outbound angle, looking again at triplets to check they do curl up.

Now remove the middle one of each triplet. Physically you'd do this by drawing non-reflecting stripes on the mirror or scratching it at regular intervals.

Removing every other path predicts increasing brightness at the detector, for this particular arrangement of source and detector. Move the detector to check it's not the case for different angles. So for some angles only, regular painting out strips of a mirror leads to increased brightness.

This effect is another surprising prediction from a photons explorations. This device, a mirror with regular, close-spaced black bands, is called a reflection grating. You can also get transmission gratings, by taking a transparent material and drawing regular, close-spaced black lines on it. You'll get the same effect: brightness at some angles only. Both kinds of grating are called diffraction gratings.