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Mirrorspin
Foucault Speed of Light / 298 000 km/s Physics
Mirrorspin, a helmeted knight, aims a spinning mirror and a bright orange beam.
The Physics
In 1862 Léon Foucault measured the speed of light in a laboratory with a rapidly rotating mirror (Comptes rendus 55, 501). Light reflects off the spinning mirror to a distant fixed mirror and back. In the round-trip time the rotating mirror turns a tiny angle, so the returning spot lands slightly displaced. From the displacement, the path length and the spin rate he got c = 298 000 ± 500 km/s, close to the modern value.
Δα = 2ωD/c
- Δα: angle the mirror turns during the round trip (rad); the returning beam is deflected by 2Δα
- ω: angular speed of the mirror (rad/s)
- D: distance to the fixed mirror (m)
- c: speed of light (m/s)
In 1850 the same method showed light is slower in water than in air, against the particle theory.
Honest caveat: The formula is the simplest geometry; turning the spot displacement into an angle needs the lens distances of the real setup. The modern defined value is 299 792.458 km/s. This is a different Foucault experiment from his pendulum.
On the sticker
Mirrorspin holds a round mirror with curved motion lines and a glowing orange beam running from it. Printed text: MIRRORSPIN with the line 298 000 km/s. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.
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Details
Size3″ × 3″
3″ × 3″ kiss-cut sticker.
Shipping
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Field notes
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