= Sagnac interferometer
{c}
A <Sagnac interferometer> uses a <beam splitter> to send <light> with <optical coherence> around a common closed optical path in opposite directions, then recombines the beams to read their <phase difference>. <Mirrors> or a guiding path define the loop; the source, <beam splitter>, loop and readout rotate together in the basic experiment. A small relative beam tilt makes spatial <interference fringes> whose displacement reveals a change of <phase difference>. At one fixed output, a <photodetector> can instead record the corresponding change in <light intensity>.
\Image[/sagnac-rotating-interferometer-geometry.png]
{title=Counterpropagating light around a rotating closed loop, with a shared beam splitter and readout}
{id=image-sagnac-rotating-interferometer-geometry}
{description=Original rectangular teaching layout. The <vector area> points out of the page for the positive counterclockwise path. The source and readout rotate with the <mirrors>. Arrow offsets separate the two directions visually; they do not represent different physical paths. The drawing is not a reconstruction of the historical mirror arrangement.}
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