The Michelson Morley experiment depicted here uses a diode laser of 780nm wavelength. The laser beam is split into two paths, one reflecting from mirror M1 and the other from M2. The beams are brought back together at the detector.

Detector Laser Beamsplitter M1 M2 d Moving the mirror M2 results in a difference in path lengths. Interference of the beams at the detector is destructive if the difference in the pathlengths of the beams is/are