I have a bachelor's in physics & its recently struck me that I do not understand, semantically, what phenomenon allows us to measure the speed of light through air in a small room with a laser and a spinning mirror. I understand "the how," which is to say I know that measuring the angle reflected by a beam (fired at a fixed distance) against a mirror (rotating at high angular velocity) allows us to measure the speed of that beam. What i do not understand is why this experimental method is possible, and therefore satisfactory to claim measurement of a finite speed.
For a typical Fizeau–Foucault method setup:
If the beam is continuous & mirror perfectly flat, shouldn't light always get reflected at whatever angle is governed by normal geometry?...as a result, I would imagine a wedge of light having fixed-width, & depending only on the spatial coordinates of the mirror, not on