It is important to know what happens when a photon hits the surface of the mirror and then reflects to pass on the segment C. Normally at the point of impact on the mirror the speed of the photon becomes zero in a period of time that we cannot determine although this period of time can be a new constant in physics then the photon leaves to travel the segment C but at the end the average speed of the photon is a constant which is, c along the route B+C. Normally the average speed of the photon on its path B and C must be less than, c in classical physics. The question is what must be the behavior of the photon so that its speed remains equal to, c yet it stopped for a period of time at the moment when it hit the mirror to be reflected and leave again on the segment C?
My answer is that in my opinion the photon has a temporal component which can be modified under certain conditions! In my opinion in our present example the photon of the B+C segment undergoes a temporal dilation to compensate for the loss of speed undergoes when the photon struck the mirror!