Let's take a particle with the speed of light and throw it to a reflective surface with a 90 degree angle. We know that the speed of light is constant thus this particles speed is constant. But doesn't it's speed reduces to 0 at the fraction of reflection ? If it accelerates, doesn't this phenomenon contradict with Einstein saying speed of light is a constant ? (I know that light can accelerate due to its form of wave therefore I used I single particle at the speed of light.)
Maybe this is not the answer but the concept is light has no mass and has a constant speed c in vacuum, if you change the medium you can observe less than that but not more. If you wonder that how does acceleration effects the photon, maybe you can see gravitational shift (red shift), in this case velocity cannot change but the frequency can. That is, no contradiction.