I'm reading about an experiment done with a Michelson interferometer. The aim is to measure the thickness of a piece of plastic. They use white light, so the central fringe in the interference pattern, corresponding to equal path lengths in the two beams can be used as a reference. Why does that work? I'd really appreciate an explanation (as simple as possible) as to why they do this. The central fringe corresponds to both beams having travelled equal distances... The displacement of one fringe has to be tracked when the plastic is placed. That's about as much as I understand. Why is the central fringe a reference? If it's the only one that doesn't move, why is that? Thanks for any help!