Firstly, I must apologise if this thread is in the wrong place on the forums. I wasn't entirely sure where it might suit best. My question arises from something taken pretty much as read without much thought. That light travels through varying edia at varying speeds. I am curious as to why this is? In the case of massive objects on larger-than-elementary scales, the concept of movement through matter being slowed make sense. Swimming, for example, there are water molecules that must be pushed aside, whereas in the air, there are more 'gaps' between molecules and the air molecules are 'easier' to push aside. Yet for subatomic particles such as photons, passing through water, any incidents of contact would result in certain energy exchanges, whereas the photons that do not enter any contact or exchanges with particles would simply continue on unabated. Is there simply a case where all (or an overwhelming majority) of photons DO collide and are then scattered and/or re-emitted, and it is the time taken between various particles emitting and absorbing photons in succession that results in the longer time takemn, therefore a slower OBSERVED speed, or perhaps, multiple scattering causes a greater distance until the photons finally reach the observer, again, indicating a slower OBSERVED speed? Even if the two possibilities I could think of outlined above are responsible, it still implies a speed of 'c', where only the distance or time taken for an individual photon is inadequately assumed, so the result of a slower speed is in fact, incorrect.