When a photon changes mediums it's speed wrt to c changes correct? So dv/dt = a (d as in delta). So how long does it take a photon to "pass through" one medium into another, and is this considered acceleration. Also if it doesn't "pass through" one medium to another(meaning it isn't like shooting a bullet into water, instead it just "is") then how CAN you determine the dv if there is no dt? And who accelerates? The photons or the medium?