Past exam of the physics A course of the University of Cambridge / 2018 / Paper 2 / Section B / B8 / b / Solution

The beam splitter sends transmitted and reflected portions into perpendicular arms. Each mirror returns its beam to the splitter, where the two returning fields recombine towards the detector. A collimating lens provides approximately plane illumination. A compensating plate of the same glass thickness as the beam splitter can equalise the dispersive glass paths, which matters especially for broadband light.

If the arm lengths are ${\ell }_1,{\ell }_2$, the geometric optical path difference in air is $x=2({\ell }_1-{\ell }_2)$, up to a chosen constant and any additional material paths. Moving one mirror by $d$ changes $x$ by $2d$. Fixed reflection phases can be absorbed into the reference position of a bright or dark fringe.