According to Special Relativity, A particle with rest mass = 0 can exist if and only if it is moving at the speed of light. A photon is a particle which comes into this category. However, when light travels in a medium it's speed is always reduced. Does this contradict relativity? What I want to ask is that when light travels at a lower speed in some medium. do photons exist and why? Why do photons 'seem' to exist at speeds less than that of the speed of light.