In this graphic you can see a sequence* of three photons in the medium of air on the left. All photons are at the same distance from each other. Since the light processes flash one after the other with a constant frequency of the respective light, a constant speed of light results. When light enters a glass body, the distance must become smaller because the speed of light in glass is smaller than in air. This reduced distance is shown in the graphic for the three middle light processes. But how does the photon then “know” the correct distance? This knowledge lies as a law in the non-manifest potential, which is the common source for all processes in the universe. And because each photon flashes again and again from this common source, the information for the appropriate distance for each new photon is always directly available. The light also “knows” that when it leaves the glass body it has to switch back to the larger photon distance so that the light has the right speed for the medium air again after leaving the glass body.
 

At this point it is perhaps appropriate to mention that models like the quantum model are just parables, and not a statement that this is how reality is. Parables and models can point to important aspects of reality, make connections clearer and thus help to better understand reality. In this respect, the quantum model can be helpful.

* This graphic shows all light processes in a sequence, each of which flashes energy (wave) and photon. However, additional conditions are still needed to flash a photon. They only flash when light processes interact, for example with a measuring instrument or an eye.