The Fizeau experiment, conducted by French physicist Armand Fizeau in 1850, was one of the first experiments to measure the speed of light in a terrestrial setting. Fizeau set out to determine the speed of light by employing a rotating toothed wheel and a light source.
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In 1851 Hippolyte Fizeau detected a change in light propagation through moving water using interference fringes. Sunlight entered a telescope through a partially reflecting mirror; two slits selected beams passing through parallel glass tubes, each long with internal diameter. A second telescope and focal mirror exchanged the tubes on return, and the beams recombined at a graduated eyepiece. Compressed air drove water between reservoirs, producing opposite flows; reversing both flows reversed the interference fringe displacement. The double transit suppressed static differences in temperature and pressure; filling only one tube with water tested this compensation. Separate supports reduced mechanical disturbance. Hippolyte Fizeau estimated flow speed from volumetric flow rate divided by tube cross section.[ref]
The Hypotheses Relating to the Luminous Aether by Hippolyte Fizeau. 1851 report, apparatus and compensation paragraphs; flow production, independent supports and fringe observations; scanned translation pp. 569–571
The Fizeau experiment turns a tiny change in phase velocity into a visible fraction of an interference fringe. The Fizeau experiment path-length convention below explains why both tube passages contribute with the same sign for one beam and the opposite sign for the other. This makes the apparatus sensitive to the part of the phase difference that reverses with the flow, while rejecting disturbances shared by both beams.
