Fizeau experiment
![Apparatus used in the Fizeau experiment](/Images/godic/202501/13/Fizeau-Mascart25027.png")
![Improved Fizeau type experiment by Michelson and Morley in 1886. Collimated light from source a falls on beam splitter b where it divides: one part follows the path b c d e f b g and the other the path b f e d c b g.](/Images/godic/202501/13/MM86-55027.png")
![Hoek expected the observed spectrum to be continuous with the apparatus oriented transversely to the aether wind, and to be banded with the apparatus oriented parallel to the wind. In the actual experiment, he observed no banding regardless of the instrument's orientation.](/Images/godic/202501/13/HoekExperiment_with_expected_results5027.png")
The Fizeau experiment was carried out by Hippolyte Fizeau in 1851 to measure the relative speeds of light in moving water. Fizeau used a special interferometer arrangement to measure the effect of movement of a medium upon the speed of light.
According to the theories prevailing at the time, light traveling through a moving medium would be dragged along by the medium, so that the measured speed of the light would be a simple sum of its speed through the medium plus the speed of the medium. Fizeau indeed detected a dragging effect, but the magnitude of the effect that he observed was far lower than expected. His results seemingly supported the partial aether-drag hypothesis of Fresnel, a situation that was disconcerting to most physicists. Over half a century passed before a satisfactory explanation of Fizeau's unexpected measurement was developed with the advent of Albert Einstein's theory of special relativity. Einstein later pointed out the importance of the experiment for special relativity.