Speed of Light
Historical methods to measure c. Builds on Traveling EM Waves.
In 1849, Hippolyte Fizeau set up a brilliant experiment: he aimed a beam of light through the gaps of a rapidly spinning toothed wheel, toward a mirror 8.6 km away. The light passed through a gap, traveled to the mirror and back (17.2 km total), and returned to the wheel.
How did Fizeau actually measure the speed of light with this setup?
At a critical wheel speed, the light returns to the wheel just as the next tooth has moved into position — blocking it! Increase the speed further, and the light arrives as the NEXT gap aligns — letting it through again. From the known distance, number of teeth, and the critical RPM, Fizeau calculated c ≈ 3.15 × 10⁸ m/s — only 5% off!
Loading simulation…
Adjust the wheel speed and watch the light pulse travel to the mirror and back. Find the critical speed where the returning light gets blocked!
c = 4D × N × f
The light must travel 2D (round trip) in the time it takes the wheel to turn half a tooth spacing (1/(2N) turn) — that’s when a tooth first blocks the return. So c = 2D/(1/(2Nf)) = 4DNf.
Even at c, signals to Mars take 4 to 24 minutes depending on orbital position. Real-time remote control of rovers is impossible.
Light in glass fiber travels at about 2/3 of c. A signal from New York to London (~5,500 km) takes about 27 milliseconds.
The finite speed of light means telescopes are time machines. The Andromeda galaxy’s light took 2.5 million years to reach us — we see it as it was then.
GPS relies on the precise speed of light to triangulate position. A 10-nanosecond timing error means a 3-meter position error.
“Measuring c was one of humanity’s greatest experimental triumphs — from spinning wheels to laser beams, we pinned down the universe’s speed limit to the last digit.”
Speed of Light Lab
Recreate Fizeau’s experiment. Adjust the wheel speed, number of teeth, and mirror distance. Find the critical RPM where the light gets blocked, then use it to calculate c. Compare with Foucault and Michelson’s methods.
Loading simulation…
Start with Fizeau’s actual parameters: 720 teeth, 8.6 km to mirror. Slowly increase wheel speed until the return light disappears — that’s the first blockage. Note the RPM. Now double it and the light reappears through the next gap! Try changing the distance — how does the critical speed change?
Fizeau’s method is a beautiful example of how cleverness overcomes limitations. You don’t need a timer that runs at 10⁹ ticks per second — you just need a wheel with enough teeth spinning fast enough.