I have a question about measuring the speed of light. As I understand it is done by sending photons from A at time t1 to B. B is at a distance d from A. At B there is a mirror, which sends the photons back to A. At A their arrival time is measured as t2. The velocity then is calculated by (t2 – t1) / 2d. But is this calculation complete? At B there is a mirror, which changes the direction of the photons. The calculation assumes that the change of direction for the photons takes no time. But is this really the case? At B the photons have to interact with the matter of the mirror for changing their direction. How fast does it happen? I suppose that this time span (if there is any) for light to change its direction is very small compared to the accuracy of the measurement of t1 and t2 and d, or else we would calculate different speeds of light for different distances d between A and B (the longer d, the less important the time necessary for changing the direction). Are these thoughts correct? Thank you.