The difference velocity works, difference in time travelled works, but considering a waveLENGTH is given, and a LENGTH is asked, why convert twice? But your answers and logic are correct, so it's not really a problem.
An interesting question for you: I have a parallel light beam, and I block the center of the beam by an object. But right behind this object (again in the center of the beam) I still find light. What effect is causing this?
The name of the physicist that thought this was impossible is actually the name of our student union. This spot is named after the physicist that first observed it.
