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Fizeau's Moving Water
Plate 026 Physics Poster
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.
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Field notes
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