1. The problem statement, all variables and given/known data In an inertial reference frame, a beam of light is shone 30 degrees from the x-axis. What is the speed of another inertial reference frame along the x-axis where the beam of light is 90 degrees from the x-axis? 2. Relevant equations Can't really think of any equations that would be applicable to this. 3. The attempt at a solution My reasoning is that this is impossible, which is an option. The speed of light is c in all inertial reference frames, and to the observer in the primed frame, the light would not have its trajectory changed as it is always going at c with respect to them. One of the options is c, which I can understand being the answer as the light would not be moving with respect to the observer in the x-axis, and would have to be in the y-axis. But isn't this impossible - no reference frame can move at c?