while searching for different methods of speed of light measurement, i came across one of the method of 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 measure 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 soo many light particles leaving and entering different holes (or teeth)?

  • $\begingroup$ Cross-posted on Astronomy. $\endgroup$
    – HDE 226868
    Commented Nov 28, 2014 at 15:01

1 Answer 1


This image:

fizeau speed of light arrangement

is the most explicit one I've found for the experimental arrangement. Here's another that is more schematic, but illustrates the basic idea:

fizeau speed of light arrangement

Now on to your specific questions. The light passes through the gaps between the teeth; it is not refracted. Pulses of light hit the mirror and are reflected back.

In the experiment, the speed of the wheel is gradually increased. Initially the light making it back through the toothed wheel gets darker and darker. That's because the light pulses arrive in time to hit the teeth instead of the gaps between. As the speed of the wheel is increased, the reflected light starts to get brighter, reaching a maximum when the timing is perfect: a pulse leaves the gap on the way to the mirror, reflects, and makes it back just in time to pass through the next gap. In principle, as the wheel speed is increased, the experimenter should see the reflected light getting darker, then lighter, then darker, etc. Thus, the second brightening should occur when the returning pulse of light goes through two gaps past the original gap.

Here are some more historical details, from the MacTutor biography:

In July 1849 Fizeau set up a mirror at his parents' home at Suresnes and another on Montmartre, the hill on the right bank of Paris. These were 8633 metres apart. Between them he set up a rapidly rotating toothed wheel and determined the speed of rotation necessary for the time taken by the light travelling between the mirrors to equal the time taken for the wheel to rotate by one tooth. He was thus the first to make a successful terrestrial measurement of the velocity of light. We know today that the value he found was almost as accurate as Römer's astronomical measurement having an error of about 5%.


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