I asked these questions because I wanted to clearly understand this experiment. The first bewilderment was why "mirror"? The difference between a surface matt and mirror is the reflected light directionality. Doppler effect to happen for a mirror, must necessarily be a movement that is not in its own plan. A mirror is placed on a table where he moves, does not have Doppler effect. No, wrong, and so there is Doppler effect. The measurement can not be when the light source and receiver are in the same place. This is not true for diffuse reflection. On the perimeter of a wheel, the movement is tangent to the wheel, is in the same plane as a "surface" of the perimeter. If this perimeter is a mirror, you can not measure the Doppler effect, in the case when the light source is in the same place with the receiver. I've found something interesting! Please Register or Log in to view the hidden image! Doppler_effect It reminds you of something?