= 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 (optics)> 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.
\Image[/past-exam-of-the-physics-a-course-of-the-university-of-cambridge/2018/paper-2-b8-michelson-layout.png]
{title=Michelson interferometer with a movable mirror, detector, beam splitter and compensating plate}
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.
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