One of the main postulates of relativity is that the speed of light is always constant. However, light does not always travel at c, it only travels at that speed in a vacuum (supposedly). When it is in a medium such as water or air, it moves at a noticably slower rate, where even other objects are capable of moving faster than light (not as eye-popping as it sounds). But do the same rules of relativity apply in this case? If not, how should light be treated in this case?