I heard once in a TED talk how Fizeau measured the speed of light in the 19th century. Here is the link


You can read about it here in Wikipedia:


A short summary: He placed a kind of rotating wheel in front of a beam of light, and a mirror far away from these two things. The beam of light passes between two teeth of the rotating wheel, reaches the mirror and goes back from the original source. As the wheel is spinning very fast, during the time that the light has been travelling, the wheel has rotated a tiny bit, but enough to impede the passage of time through the point where it entered. Knowing the distance from the mirror as well as the speed at which the wheel is rotating, the speed of light can be easily calculated. The experiment is better explained in wikipedia, here I wrote a simplified version of it.

I loved the experiment, because it seemed fairly easy to reproduce, so I ordered a green laser pointer on Amazon, which can reach up to 10 km. As a proof of principle, I went with a friend in the night to a place where there is a good visibility. We began setting a mirror somewhere 500 metres away from the laser, but, even from that far, the light had scattered so much that it was impossible to collect the light with the mirror.

The laser is a very powerful one, of those that you can see the whole beam (usually used in astronomy). If I can't repeat the experiment using a laser like this, how on earth could Fizeau do that in the 18th century employing a much more rudimentary source of light and placing the mirror much farther away? It says in Wikipedia that the distance between them was like 8 km.

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    $\begingroup$ So what is your question? How to reproduced the experiment. $\endgroup$ Aug 15, 2014 at 19:57
  • $\begingroup$ Hopefully the laser came with some specs. Not just power output (btw you were lucky to get an actual high power one from amazon - most of the lasers online are many times worse than advertised) but also "beam divergence" or something similar. That could at least help in diagnosing why your particular setup didn't work. $\endgroup$
    – user10851
    Aug 15, 2014 at 20:48
  • $\begingroup$ Was your particular issue that the reflected light was unstable? en.wikipedia.org/wiki/Corner_reflector Without lasers, I'd use more fire. $\endgroup$
    – user121330
    Aug 15, 2014 at 21:36
  • $\begingroup$ Part of the problem seems to stem from the fact, that you didn't actually try to repeat Fizeau's original experiment, but you tried to create your own "ad hoc" version of a similar one. So what really failed was your experiment, which was not planned well. Did you think about collimating your light source, how did you assure the quality of the mirror? $\endgroup$
    – CuriousOne
    Aug 16, 2014 at 0:54
  • $\begingroup$ If you could see the beam spot in the distance that means that at least some light already came back to your eye! And that might (at least in principle) just be enough. The mirror and potential lenses just serve to enhance the amount of light that comes back. There's no requirement to get the whole beam back. $\endgroup$
    – Emil
    Mar 23, 2015 at 10:14

1 Answer 1


There is a much better description here of Fizeau's nineteenth century experiment. Some of the key features that enabled Fizeau to succeed:

  • A lens to collect the light from the source
  • A collimating lens to prevent the light diverging during its journey
  • A large diameter beam to minimise broadening of the beam by diffraction
  • More lenses to focus the light on the detector
  • The light went directly into a very sensitive detector: the human eye. Almost certainly he did this experiment at night.
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    $\begingroup$ Minor comment to the post (v1): Please consider to mention explicitly author, title, etc. of link, so it is possible to reconstruct link in case of link rot. $\endgroup$
    – Qmechanic
    Aug 16, 2014 at 7:35

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