In this graphic you can see on the left a sequence* of three photons in the medium of air. All three 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 then enters a glass body, the speed of light is reduced. 
This is shown as reduced distance between the three middle light processes in the graphic. But how does the photon then “know” the correct distance? This knowledge lies as a law in the non-manifested potential, which is the common source for all light processes in the entire 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. When leaving the glass body the light also “knows” that 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.