# Relation Between Speed of Light & Reflected Angle (Fizeau–Foucault)

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 t.

To help try and make my question clearer, I made a diagram. The red arrows are how I would expect light to be reflected off of a spinning flat mirror:

• Wikipedia has a good explanation of the experimental setup. Dec 7 '18 at 8:45