Fizeau's Moving Water

Plate 26 speaks for itself: “Water drags light, but only 44%.”

The Physics

Fizeau timed light sent with and against water flowing in tubes. The water dragged the light, but only by Fresnel's partial-drag fraction, a result special relativity later explained as velocity addition.

u ≈ c/n + v (1 − 1/n²)

• u: speed of light in the moving water (lab frame), m s⁻¹
• c: speed of light in vacuum, m s⁻¹
• n: refractive index of water, dimensionless
• v: water speed, m s⁻¹

Paris, 1851: about half of full drag. For n = 1.333, 1 − 1/n² ≈ 0.44.

Sources

• H. Fizeau, C. R. Acad. Sci. 33, 349 (1851).

On the poster

Plate 026, “Fizeau's Moving Water”: the pixel-art plate with its slogan “Water drags light, but only 44%.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.