Talk:Foucault's measurements of the speed of light

Rotating mirror
" As the rotating mirror"


 * it says "rotating strobe disk" in the image. which is it? - Omegatron 18:27, May 25, 2005 (UTC)
 * The explanation with the rotating mirror is about the "Fizeau-Foucault apparatus" depicted in the second image, and not about the original Fizeau, depicted in the first.--Laur2ro | Talk 15:49, 28 August 2007 (UTC)

Speed of light in moving media
" According to the non-relativistic view at the time of the experiment, the speed of light ... is ... "

The section then gives an "expected" equation for lightspeed in moving water and contrasts it with the "surprising" actual result.

Except ... I thought that the "surprising" result was the one that Augustin Fresnel had already predicted (some time around 1818), and that Fizeau had actually set out to find. I thought that this experiment was supposed to be looking to check Fresnel's predictions for the velocity of light in moving media, and Fizeau's result had seemed to confirm Fresnel's prediction to the available experimental accuracy (see Aether_drag_hypothesis ).

I think the usual historical account of the experiment is that Fizeau set out to test Fresnel's prediction, reported a success, and that later, Einstein argued that special relativity could also be used to generate the equation, as a first approximation. But Einstein's "low-velocity", "first approximation" equation for SR seems to be identical to Fresnel's.

Now, I understand that it's tempting for the SR guys to do a little bit of history-rewriting so that the story makes their theory look better ("Experiment refuses to agree with then-current theory, scientists are taken aback, then SR comes along to save the day"), but really, that's not what more contemporary sources say happened.

At the very least, the section should include a mention of Fresnel, after all it was supposed to be his theory that was being tested here. ErkDemon (talk) 02:29, 3 April 2010 (UTC)

WikiProject Engineering High-Importance Rating: Justification?
According to WikiProject_Engineering/Assessment, a "high-importance" article is "The article is about the basic technologies and infrastructures or the most well-known or culturally or historically significant aspects of Engineering." Can someone provide justification for this article's "high-importance" rating? I understand the importance of the device, but I don't believe it is well-known enough to justify a "high-importance" rating. I've changed it to "mid-importance" unless someone can provide justification. --Charlesreid1 (talk) 00:18, 4 November 2010 (UTC)

Partial merge of Fizeau–Foucault apparatus to Fizeau interferometer
The Fizeau-Foucault apparatus for determining the speed of light is not the same as the Fizeau experiment for determining the effect of moving media on the speed of light, which used an interferometric method. Hence, a partial merge was necessary:

Stigmatella aurantiaca (talk) 11:41, 15 April 2012 (UTC)

Sentence Does Not Make Sense
I read the article with the following sentence:

"The detector is at an angle 2θ from the source direction because the normal to the rotating mirror rotates by θ, decreasing by θ both the angle of incidence of the beam and its angle of reflection."

Is it me or does is seem as though there is a word missing where "normal" is the modifier? I dare not correct this myself as I have no previous knowledge of the topic discussed herein, that being the Fizeau apparatus and his experiment. — Preceding unsigned comment added by 2602:306:320A:AF0:8998:1E50:B266:C06D (talk) 19:09, 28 June 2015 (UTC)


 * I agree with you that the sentence is nearly incomprehensible. The word "normal" in this context means "a line perpendicular to the mirror", so the sentence is technically correct, but it is pretty awful if you aren't familiar with the technical terminology. I'll see what I can do to fix the explanation. Thanks for pointing out how bad the explanation is! Stigmatella aurantiaca (talk) 22:30, 28 June 2015 (UTC)


 * I appreciate that someone is tackling this heady subject and making it comprehensible.2602:306:320A:AF0:1DD3:EA3:26E7:167F (talk) 03:54, 29 June 2015 (UTC)


 * It's going to take a bit of research to fix this article. I not only will have to replace the text, I will have to replace the drawing, which leaves out essential elements. Meanwhile, I recommend that you read Project Gutenberg's transcript of Experimental Determination of the Velocity of Light (1878), by Albert Michelson. Michelson describes his refinement of Foucault's measurement with great clarity. Stigmatella aurantiaca (talk) 05:18, 29 June 2015 (UTC)


 * I'm still working on it, but how is the article so far? In the next several days, I hope to be adding a section on Michelson's 1877 refinement of Foucault's measurement. Stigmatella aurantiaca (talk) 12:43, 1 July 2015 (UTC)

General or Specific?
I'm a little confused. Is "Fizeau-Foucault apparatus" a general terms for a group of related devices, or a specific one? The article begins with:

The Fizeau–Foucault apparatus (1850) (Figure 1) was designed by the French physicists Hippolyte Fizeau and Léon Foucault for measuring the speed of light.

This seems to say that this article describes a specific device, and indeed goes on to describe its operation (one I am very familiar with). But throughout the article many other devices by these people are described as well, with no indication of whether they are simply related apparatuses or actually fall under the category "Fizeau-Foucault Apparatus." Is Fizeau-Foucault apparatus then a general term for devices created by Hippolyte Fizeau and Léon Foucault that have to do with light, or a specific one of those devices? --ValekHalfHeart (talk) 05:42, 9 January 2012 (UTC)

History of term "Fizeau–Foucault apparatus"
Is there any evidence that the term "Fizeau–Foucault apparatus" ever existed in the literature prior to its appearance as the title of this article, which was created in 2002? Stigmatella aurantiaca (talk) 04:52, 26 July 2015 (UTC)


 * The short answer is yes, on page 192 of "Introduction to Geometrical and Physical Optics" by Joseph Morgan, 1953. Despite that I don't think it is the right title for the Wikipedia article, and other editors agree with me, see above. It conflates the two quite different experiments. I intend to split the article, by creating a new article for Fizeau's experiment. cagliost (talk) 21:03, 9 March 2023 (UTC)

Article should be split (subject not clear)
It's really not clear what is meant with "Fizeau-Foucault apparatus": the later rotating mirror experiment usually attributed to Foucault, or a generalized term that incorporates both experiments, or ...?

Most other language Wikipedias (e.g. de) distinguish between rotating cog wheel (Fizeau) and rotating mirror experiment (Foucault) and provide separate articles. — Preceding unsigned comment added by 37.49.73.197 (talk) 10:14, 25 April 2015 (UTC)


 * That's a very good idea, and the German-language articles would form a good basis for the new English-language articles. Then the Fizeau–Foucault apparatus page could be converted to a DAB page. Stigmatella aurantiaca (talk) 06:53, 26 April 2015 (UTC)

Unfortunately it's not true that most other Wikipedias have separate articles for the two experiments. German does, and now English, but most other languages use the "Fizeau-Foucault apparatus" abomination. cagliost (talk) 01:03, 10 March 2023 (UTC)

Article has now been split
New article is Fizeau's measurement of the speed of light in air. Existing article moved to "Foucault's measurements of the speed of light".

The term "Fizeau–Foucault apparatus" was rare before it was popularised by Wikipedia. It's not a good name for the spinning mirror experiment, because Fizeau's contribution is apparently negligible: the idea came from Wheatstone and Arago, and the experiment was carried out by Foucault.

In any case, the two experiments (spinning mirror versus toothed wheel) are quite different, and putting them in the same article was evidently confusing readers, especially since Fizeau's toothed wheel experiment was simply about measuring the absolute speed of light (in air), whereas Foucault's 1850 experiment was about the relative speed of light, and the wave or particle nature of light.

If a reader wants an article which describes both experiments together, they can go to the article on the speed of light, which describes various experiments to measure the speed of light.

cagliost (talk) 23:25, 9 March 2023 (UTC)


 * Turns out Fizeau did work with Arago to carry out this experiment in 1850, but Foucault beat them to it. Nonetheless, Fizeau's 1848-1849 experiment using the toothed wheel is completely separate and belongs in its own article. cagliost (talk) 16:44, 10 March 2023 (UTC)

"Fizeau–Foucault apparatus" listed at Redirects for discussion
The redirect [//en.wikipedia.org/w/index.php?title=Fizeau%E2%80%93Foucault_apparatus&redirect=no Fizeau–Foucault apparatus] has been listed at redirects for discussion to determine whether its use and function meets the redirect guidelines. Readers of this page are welcome to comment on this redirect at  until a consensus is reached. Tevildo (talk) 14:43, 6 January 2024 (UTC)