# Talk:Foucault's measurements of the speed of light

" 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)

" 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#Historical_importance ).
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)

According to Wikipedia: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)

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)

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)

This review is transcluded from Talk:Fizeau–Foucault apparatus/GA1. The edit link for this section can be used to add comments to the review.
Reviewer: Cirt (talk · contribs) 23:00, 24 October 2015 (UTC)

I will review this article. — Cirt (talk) 23:00, 24 October 2015 (UTC)


### Image review
File:Fizeau.JPG = hosted on Wikimedia Commons = licensing checks out upon my Image review of the page there, no issues here. Y
File:Michelson's illustration of the Foucault experiment - annotated.png = hosted on Wikimedia Commons = licensing checks out upon my Image review of the page there, no issues here. Y
File:Speed of light calculation using Foucault's rotating mirror.png = hosted on Wikimedia Commons = licensing checks out upon my Image review of the page there, no issues here. Y
File:Michelson's 1879 Refinement of Foucault.png = hosted on Wikimedia Commons = licensing checks out upon my Image review of the page there, no issues here. Y
No issues with Image review. Next, on to Stability assessment. 
— Cirt (talk) 23:51, 24 October 2015 (UTC)


### Stability assessment
Upon my inspection of article edit history, article is stable going back at least to July 2015.
Article talk page shows good deal of discussion but appears to be positive and collaborative in nature.
No issues here. Next, on to rest of review. — Cirt (talk) 00:27, 25 October 2015 (UTC)

Question
While writing this article, I had debated whether I should add a citation to Sur les vitesses relatives de la lumière dans l'air et dans l'eau / par Léon Foucault (1853) into the section on Fizeau's determination of the speed of light, and a citation to Détermination de la vitesse de la lumière: d'après des expériences exécutées en 1874 entre l'Observatoire et Montlhéry, by M. A. Cornu (1876) into the section on Cornu's refinement of the Fizeau experiment. I ultimately decided against adding the citations because the references are in the French language, but in leaving them out, the two sections have fairly extended sections of otherwise unsourced text. Was this a mistake? Thanks! Stigmatella aurantiaca (talk) 14:01, 26 October 2015 (UTC)

Yes, please add the citations. Every single fact, and ideally ever single sentence, or at least ends of every single paragraph, should have in-line citations. You can use WP:CIT templates, and specify parameter for language. — Cirt (talk) 02:06, 27 October 2015 (UTC)
Added the Cornu and Fizeau references. Stigmatella aurantiaca (talk) 03:27, 27 October 2015 (UTC)
Decided against the Fizeau reference, used Hughes 2012 instead. Stigmatella aurantiaca (talk) 03:44, 27 October 2015 (UTC)


### Good article nomination on hold
This article's Good Article nomination has been put on hold. During review, some issues were discovered that can be resolved without a major re-write. This is how the article, as of October 28, 2015, compares against the  six good article criteria:

1. Well written?:
Thank you very much for your efforts to contribute to Quality improvement on Wikipedia, it's really most appreciated !!!
NOTE: Please respond, below entire review, and not interspersed throughout, thanks!
Suggestion: This suggestion is optional only, but I ask you to please at least read over the Good Article review instructions, and consider reviewing two to three (2-3) GA candidates from good articles nominations, for each one (1) that you nominate. Again, this is optional and a suggestion only, but please do familiarize yourself at least with how to review, and then think about it. This is a way to help out the Wikipedia community by reducing our GA Review WP:BACKLOGS, and a form of paying it forward. Thank you !
Copyvio Detector - https://tools.wmflabs.org/copyvios/?lang=en&project=wikipedia&title=Fizeau%E2%80%93Foucault+apparatus&oldid=&action=search&use_engine=0&use_links=1 - shows no major issues - GREAT JOB HERE, THIS IS WHAT WE LIKE TO SEE, NICE !!!
Suggest changing Notes sect to Footnotes, as this is more extra info than notes to back up the references sect itself.
Overall, the writing quality is good, accessible to the reader as I've mentioned, below, and quite educational on a topic in science. Well done overall.
2. Verifiable?:
Per WP:LEADCITE, cites not needed in lede intro sect, as long as same exact info is cited later on in article body text.
Checklinks analysis = http://dispenser.homenet.org/~dispenser/cgi-bin/webchecklinks.py?page=Fizeau%25E2%2580%2593Foucault_apparatus = shows at least one problem in blue = "Service unavailable".
Please try archiving that link, and as many other links as possible, using Wayback Machine by Internet Archive via WP:CIT template fields archivedate and archiveurl.
Please add in-line-citations to the ends of each of the Footnotes, to back up the factual assertions made therein.
Strongly recommend adding in-line-citations to the ends of each of the captions in the images, to back up info asserted in those sentences.
3. Broad in coverage?: The article is indeed thorough with a good overall presentation and structural layout. It is an esoteric science topic and yet written in an accessible manner for the reader, albeit obviously slightly clearer for those readers with some prior physics instruction as background.
4. Neutral point of view?: The article appears to be neutral and is written in a matter-of-fact tone, throughout.
5. Stable? See above, Stability assessment, no issues here.
6. Images?: See above, Image review, no issues here.
NOTE: Please respond, below entire review, and not interspersed throughout, thanks!Please address these matters soon and then leave a note here showing how they have been resolved. Within 7 days, the article should be reviewed again. If these issues are not addressed by then, the article may be failed without further notice. Thank you for your work so far. — Cirt (talk) 04:54, 28 October 2015 (UTC)


### Responses
Re 2. Verifiable? Per WP:LEADCITE Not all info in lede is repeated in main body, so citations are necessary. I do not want to go into detail in the main body explaining the origin of the confused term "Fizeau–Foucault apparatus", because my WP:OR research indicates that the term was a neologism invented by a currently blocked Wikipedia editor in 2002 who didn't know the difference. Nor does going into detail about Fizeau and Foucault's partnership and breakup seem entirely relevant to the main focus of the article.
Re 2. Verifiable? Checklinks analysis Archiving is partway completed. Wayback Machine failed to properly archive several pages. Will try WebCite as an alternative tomorrow. Stigmatella aurantiaca (talk) 06:02, 29 October 2015 (UTC)
WebCite would not archive the links in question either, but was a bit ambiguous about the reason. So archiving is complete. Stigmatella aurantiaca (talk) 11:44, 29 October 2015 (UTC)
Re 1. Well written? Notes sect to Footnotes Done.
Re 1. Well written? in-line-citations to the ends of each of the Footnotes Have done so for Footnotes 1, 3, and 4. I am leaving the citation style in Footnote 2 as it previously had been, since having the identity of the source of the quote in the body of the text is almost as important as the quote itself. Stigmatella aurantiaca (talk) 12:43, 29 October 2015 (UTC)
Re 2. Verifiable? in-line-citations to the ends of each of the captions in the images Have added inline citations for Figures 2, 3, 4, and 5, since either the captions were complex or the figures were derived from original source material. The information in the Figure 1 caption is non-controversial and is unlikely to be challenged. Because of that, I do not feel that the Figure 1 caption requires an inline citation, and wish to avoid WP:OVERCITE. Stigmatella aurantiaca (talk) 13:41, 29 October 2015 (UTC)
Re 1. Well written? Suggestion ... paying it forward My reviewer count is up to 14 with one in progress. My latest completed GA as reviewer is Talk:Termite/GA1 that required nearly three weeks of work with the two principal editors.
I believe that I have addressed all of your suggestions. Thanks!  Stigmatella aurantiaca (talk) 13:55, 29 October 2015 (UTC)


### Passed as GA
Passed as GA.
GA Nominator has responded to, addressed, or explained professionally why not addressing, my suggestions, from above.
My thanks to GA Nominator for being so polite and responsive to above recommendations by GA Reviewer.
Much appreciated,
— Cirt (talk) 23:53, 29 October 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)

The redirect 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 Wikipedia:Redirects for discussion/Log/2024 January 6 § Fizeau–Foucault apparatus until a consensus is reached.  Tevildo (talk) 14:43, 6 January 2024 (UTC)

The second sentence after Figure 2: starts with "Light from a passing through...". I do not see any a in the image. I am right that there should be "Light from α passing through..."? So I mean, there should be Greek alpha instead of Latin a, in my opinion. Tomas IV (talk) 08:07, 6 March 2025 (UTC)

It has been a while since this article has been reviewed, so I took a look and noticed the following:

The lead is quite short and does not summarise all major aspects of the article.
There are several uncited statements, especially in the "Background" section.
Should this article go to WP:GAR? Z1720 (talk) 20:23, 28 September 2025 (UTC)