Special relativity states that the light speed is always fixed. This was (and is) very counter intuitive to understand, but it is what it is. For example, if you are going at a speed of 0.8c (c = speed of light) along the direction of light, then the conventional (newtonian) physics says that the relative velocity between you and the light is 0.2c only! That light appears to go slower if you are travelling faster. But special relativity says NO! Irrespective of our speed, we always measure the speed of light as 'c'. It's just that. Light speed never gonna change (in this universe). Also it's the maximum limit of anything that can move. Especially, if any particle has a mass (or to be more specific, rest-mass), then they can never reach speed of light. Now, see it in the other way around. If anything is going at less than speed of light, it should have a mass (again, rest mass) right? How about when light itself is slowing down?? Yes. Refraction. Does light gain mass ...