Start with illuminating phenomena to promote noticing and discussion, to motivate the idea of
exploring space: the arrangements of the source and what goes in the surrounding space determine the brightness at different locations – so what turns up at the detector. Discuss the space into which the photons are emitted – develop a description from the point of view of the photons, for the photons of what's out there – that enables you to predict where it's bright, and where it's not. This will be the basis of an account of lighting, of illumination, of optics, based on photons.
First do some careful noticing, starting with seeing.