The Fizeau experiment, conducted by Hippolyte Fizeau in 1851, was the first to measure the speed of light in a moving medium, such as flowing water. This groundbreaking experiment provided early evidence for how light behaves when it travels through a substance that is itself in motion.
In the experiment, Fizeau directed light through tubes of flowing water and used an interferometer to detect any change in the speed of light caused by the movement of the water. He found that the light’s speed was partially dragged by the flowing medium, but not completely—a result inconsistent with classical physics at the time.
This partial dragging effect supported the Fresnel drag coefficient, and later became a key piece of evidence in the development of Einstein’s theory of special relativity. It showed that the speed of light isn’t simply additive with the medium’s motion, challenging Newtonian views and paving the way for modern concepts of light propagation and frame-dependent physics.
The Fizeau experiment remains a landmark in both optics and relativity theory.