Summary so far.

• varying illumination is a consequence of altering the geometry
• arrange for particular lighting by manipulating the space between source and the detectors
• predict the illumination at a detector by calculating the contributions of significant paths between source and the detector

There is a big question – is this really how the photons are? To which the answer, however unsatisfying, is no-one knows. And perhaps they never could know: physics is concerned with phenomenological metaphysics – that is with building fruitful, plausible and intelligible descriptions of how quantifiable aspects of the lived-in world appear to us.