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Long ago in high school I saw a short film in which as I recall the apparatus was just a spinning paper plate (with holes along its edge) with a light source -- I don't recall a spinning mirror or a half-silvered mirror -- and the entire experiment could be done in a classroom. The basic idea that even though the speed of light was very fast you could still spin the plate fast enough to affect whether the light was visible or not struck me as very clever and almost something that could have been done by Galileo except that getting a steady speed of rotation without a motor would have been hard.

So my question is, is there a simplified version of the Fizeau experiment that could be performed within a normal-sized room with just an electric motor, pie plate and a laser pen that will give a reasonable (if less accurate than in Fizeau's actual setup) value for the speed of light or am I remembering wrongly?

Qmechanic
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releseabe
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    The original experiments by Fizeau and Foucault, with spinning mechanical devices, used light path lengths of the order of 10km. – alephzero Oct 26 '20 at 23:03
  • Maybe using a power drill? https://physics.stackexchange.com/questions/357/home-experiments-to-derive-the-speed-of-light –  Oct 26 '20 at 23:12
  • @alephzero: That's right but the film I recall was done with much shorter lengths, probably resulting in much less accuracy. – releseabe Oct 27 '20 at 00:25
  • @Pieter: I meant an experiment like Fizeau's but completely within a classroom. No microwave ovens or internet, just a steady light source and a known, possibly controllable, way of making the plate rotate. I am trying to see if the film I saw was correct and/or if I am remembering it rightly. – releseabe Oct 27 '20 at 00:28
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    The old Scientific American Amateur Scientist column had lots of expiremets along those lines. The October 1975 column was An amateur's version of A. A. Michelson's apparatus for measuring the speed of light. Index: http://amasci.com/amateur/sciamdx.html, or https://www.scientificamerican.com/author/c-l-stong/. The 10/75 issue is for sale at https://www.scientificamerican.com/article/the-amateur-scientist-1975-10/ – mmesser314 Oct 27 '20 at 00:42
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    @releseabe It's not a matter of accuracy. If you want to reduce the path length by a factor of 1000 (from km to meters) you have to make a wheel that 1000 times more teeth, (e.g. 1 million teeth not 1000) or spin it 1000 times faster. If you don't do some combination of those two things, you will get nothing. – alephzero Oct 27 '20 at 02:03
  • It would be entirely feasible to make a lab experiment to measure the speed of light directly, using the same technology as a cheap (less than $100) laser distance measuring device. But that's not what Fizeau did, of course. – alephzero Oct 27 '20 at 02:13
  • Why bother? The only reason Fizeau did it this way is because he didn't have electronics. If he had had electronics equipment, he wouldn't have messed around with wheels. The only reason to do mechanical chopping today is to recreate a historical experiment. Unless that is your aim, this is a waste of time. – FlatterMann Jun 04 '23 at 21:07

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Fizeau used a folded light path that was very long, and mirrors to bounce the beam back and forth, and a very high-speed motor running the slotted disc. There is no way to spin a paper plate fast enough in a classroom to perform the fizeau experiment with a classroom-sized baseline for the light beam.

niels nielsen
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  • The speed of the gravity-powered motor could not have been very high speed. – releseabe Oct 27 '20 at 02:51
  • I disagree: a plate (or maybe something like cardboard which is more sturdy) could be spun fairly fast and if the holes were small enough then a laser sent through a hole and reflected back through the same hole might be interfered with at reasonable speed of rotation over even very short distances: with no rotation the reflection would of course be visible but with even modest RPM the reflected beam would be affected -- so some effect could be detected. – releseabe Oct 27 '20 at 06:41
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    try doing the math first. – niels nielsen Oct 27 '20 at 10:17
  • @releseabe Even at table saw blade speeds you are talking miles of return light distance before even a dimming might occur. – Lambda Jan 31 '21 at 15:05