This thought experiment seems to have connection with the Sagnac effect. There is a windmill. At the center of rotation, a light source is fixed (and light waves are emitted as concentric circle). At the tip of one blade, two mirrors are set. Angle of two mirrors to the light source is different (like letter X). When the blade does not rotate (guaranteed by "Newton's bucket"), two reflected lights may be the same : interference fringe may not show any change (interference of two reflected lights is supposed). But when the blade rotate (light source also), how about the interference fringe (it seems to continue changing : passing)? If changing is actual, the reason may be the same to the reason on prism and laser beam that is noted above.