A laser interferometer works by bouncing light off a mirror, and detecting the change in distance by measuring the change in phase of the light when

A laser interferometer works by bouncing light off a mirror, and detecting the change in distance by measuring the change in phase of the light when it reaches back to the source, by interfering the returned beam with the original beam. A 473-nm laser is used to measure the displacement of the stage holding a chip in an electron beam lithography system. If the system can detect a change in phase corresponding to the intensity of the interference pattern at the source going from a maximum to a minimum, what is the minimum movement of the stage that can be detected?