A horizontal beam of light is reflected from a plane mirror that revolves about a vertical axis at a rate of 30 rev/min. The reflected beam sweeps across a screen that, at the point nearest the mirror, is 20 m away. With what speed does the spot of light move across the screen at the point nearest the mirror? Is it c? (3*10^8) I know I am not understanding. Can someone explain this one to me? I am not even sure where to start.