You can describe what's seen using photons. That’s useful because photons are the simplest quantum object, and physicists think the world is best described by quantum physics.
How bright a thing is depends on the number of photons a second detected and the colour depends on the frequency of the photons that arrive.
The activity (which is the number of photons second-1) at the source and detector set the power absorbed and emitted.
Photons are quantum entities, shifting energy to or from stores in definite chunks. Therefore, counting the number of photons second-1 sets the power in the beam, for a particular frequency of photon.
Change what’s between source and detector to change the number of photons arriving at the detector. (A device switches power between pathways: source, absorber and detector are all devices.) The matter that you put in the way affects what happens to the photons. What happens depends on the energy of the photon.