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While searching for different methods of calculating Speed of light, I came across one of the methods that Fizeau discussed below which I cannot fully understand.

In short, in Fizeau’s apparatus, a beam of light was shone between the teeth of a rapidly rotating toothed wheel, so the “lantern” was constantly being covered and uncovered. Fizeau had a mirror, reflecting the beam back, where it passed a second time between the teeth of the wheel.

I do understand the idea i.e. to calculate the time during the course of light "from the wheel to mirror and then back to the mirror". But where are the mirror, lantern and wheel located? What kind of wheel is that? Does light pass through the holes (or teeth) in the wheel or light gets refracted through the wheel? Finally if a group of light particles goes through one of the hole (or teeth), then how do we know that the same group of light particles came back through which hole (or teeth) after its journey, since there are so many light particles leaving and entering different holes (or teeth)?

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  • $\begingroup$ The speed of light is huge, s.t. the mirror has to be quite far from the wheel. However, the lantern has to be near the wheel. In this condition, where is it located doesn't matter, to et the speed of light we need only to measure the angle by which the wheel rotated and divide it by the angular velocity. Of course, the light passes through the holes, but I see possible errors in this method: the hole has a width, there is a distance (though small) between lantern and wheel. Probably there are remedies for these issues. $\endgroup$
    – Sofia
    Commented Nov 27, 2014 at 10:46
  • $\begingroup$ About your last question, proceed in three steps: 1) you have to start with a gross estimation of the light velocity. In this way you can get a gross estimation of the angular velocity you need. 2) for each pulse of light that you send, modified slightly the angular velocity, until you find that some pulse returns. 3) from sending one pulse to sending another pulse wait at least the gross time estimation. $\endgroup$
    – Sofia
    Commented Nov 27, 2014 at 10:57

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First: the mirror in Fizeau's original experiment was about 8km from the wheel. The light from the lantern went to a mirror at 45° so the returning light could be observed (see the picture from the german wikipedia.( as long as the wheel turn slowly you just see short light pulses coming through the slits , when the wheel turns faster the returning light hits a spoke so no light comes back to the mirror. If you double the speed you see light again since it comes through the next slit. the historical instrument is depicted in the german wikipedia: https://de.wikipedia.org/wiki/Zahnradmethode enter image description here

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