Hi Arjen.
It's 'model' rather than 'theory' (DFM), as it's highly testable.
Yes, in fact it'll be a good test to 'think on my feet' here, so here's some thinking aloud on refraction:
We're considering signal waves. A couple of 'warm up' points; Although the earth is spinning, light entering the sea on each side of the planet, dawn and dusk, is measured at exactly the same speed. This is a paradox of SR, but not when adjusted by DFM; Whatever local field the waves travel through they do it at the constant for that material ('c' or less), whatever speed that 'chunk' of material is doing in relation to any other.
Let's first consider light coming OUT of water. Say an aquarium with glass sides. It instantly speeds back up to 300,000km/sec! What amazing energy the zero mass photons must have to do that! Of course that's nonsense 19th century science. The model tests if the energy keeping light at exactly the same speed (for over 10bn years!) comes from the dark energy/matter field itself, which only allows it to travel AT 'c' and modulates it's frequency accordingly (FM is done by 'particle' oscillation) to Christian Dopplers fomulae.
Lena Hau has also demonstrated this at Harvard, stopping & restarting wave signals.
REFRACTION Stop thinking about the surface 'bending' the light. That's just our naive description of the 'effect'. Think about the wave front, at particle level, being slowed due the material change. Take 2 oscillating particles (of the 1/137th fine structure constant of the surface of the water), side by side. A light wave, part of a sphere representing an 'event', hits them at 'c' at a 45 degree angle, so it hits one particle before the other. They both slow the signal down the same amount.
Now you must remember simple causality. Our only definition of the 'angle' of the light is that the identical signal, from the part of the sphere representing that 'event', is in line with another part. if we 'draw a line' connecting the 'identical moment' signal from that sphere the line will be at an angle to the original sphere tangent. The angle is proportional to the speed change, as Snells equation.
You may need to read that twice and draw diagrams, but I assure you it works, and once you understand it you'll see how dead simple it is! Of course it does have many implications. In this case I've demonstrated the 'implications' first and just thrown your question at it as a test to see what comes out. This has been very interesting for me! and looks like a pass with Hons!! (hope you get the same), but typically easy once the basic architecture of the model's been discovered.
The real beauty is that the 1/137th increases with velocity of the mass, because the Doppler shift required is greater (also explaining the photoelectric effect). It also means the cloud of crazy free action 'photoelectrons' round a proton in an accellerator IS, or is closely related to, Dark Matter, and is the same phenomina as the dark matter halo's and shocks of larger groupings of mass. All in the viXra paper. But perhaps we're getting out of your area?
Are you starting to understand the model better now? How well could it fit with your own mental model of QM.
Best of luck with your exams.
Peter