I'm reading about an experiment done with this piece of equipment. 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 don't understand why it can be used as a reference.
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?