Talk:Arago spot

Man, I did this experiment back in college. We used a penny and taped it to a couple of strands of hair. After a bit of tinkering with the laser, the spot showed up quite clearly. I'll ask my old professor if they still do that and if they can get a photo of it. --W0lfie (talk) 17:44, 27 October 2008 (UTC)

I'd like to expand this article throughout the next month. I have authored two scientific articles on the Poisson spot with matter waves. I will add an image of the schematic experimental setup, a simulated image and hopefully much more. Treisinger (talk) 10:10, 16 June 2011 (UTC)

Poisson's spot
Why is this article titled "Arago spot" rather than its usual name? Cesiumfrog (talk) 01:37, 11 April 2009 (UTC)

I agree, the page should be called "Poisson Spot". Historically, it was Poisson who deduced it from Fresnel's wave theory, eventhough he did not believe the theory to be correct. Arago did the experiment and gave convincing proof to Poisson that the theory is actually correct. However, the Poisson Spot had been observed before a number of times, so I think the deduction from theory is the more significant contribution. Treisinger (talk) 10:10, 16 June 2011 (UTC)

The diagram  has a typo
At the bottom, please note "mulitples" for "multiples". That's an easy mistake to make. I don't yet know how to submit a corrected image, unfortunately.

Regards, Nikevich (talk) 01:56, 21 July 2011 (UTC)

Poisson's name should not be the primary name for this article
It is a shame that Poisson should usurp all the credit for the name of this phenomenon, since it was actually Fresnel that worked on the theory behind the spot, and Arago which experimentally showed it to be true. It is an injustice to Fresnel's memory to call it the Poisson spot. elle vécut heureuse à jamais  (be free) 14:18, 20 September 2011 (UTC)


 * I do not agree for the following reasons:
 * 1) The effect is called mostly Poisson Spot in the literature. I have found only one single publication refering to it as Spot of Arago.
 * 2) It is correct that Fresnel came up with the wave theory, but it was Poisson's genius that realized that there should be a bright spot in circular shadows. Arago did perform the experiment, but then it turned out that the spot had been observed before experimentally. So I think it is fair to call the spot after Poisson, since his coming up with a testable statement resulted in the ability to decide between the two pictures (wave and particle).
 * Treisinger (talk) 10:51, 11 October 2011 (UTC)


 * I'm familiar with it as the "Spot of Arago". I'm surprised to read your claim that it is mostly called the Poisson Spot in the literature.


 * All three of these eminent scientists have a valid claim to the phenomenon. It is no injustice to Fresnel to credit Poisson for being the first to predict this particular phenomenon. Arago too deserves credit for having the insight to realize that Poisson's objection was invalid, and to devise the experiment to test his claim. No doubt many lesser men heard Poisson's objection and considered the matter closed.--Srleffler (talk) 01:36, 12 October 2011 (UTC)


 * Poisson's name should be the primary name in this article because "Poisson's Spot" is the common name for the phenomenon.
 * It shouldn't be Wikipedia's function to decide who a phenomenon should be named for. The encyclopedia summarizes, it doesn't arbitrate disputes about credit. Geoffrey.landis (talk) 14:28, 17 July 2017 (UTC)
 * I agree that it's not our place to arbitrate disputes, and that the article should be named based on the most common name for the phenomenon. It is not established, however, that "Poisson's Spot" is the most common name for it. We need a reliable source. We may also need to consider whether the "most common" name depends on the context: general media, high school physics texts, undergraduate texts, scholarly literature, etc. --Srleffler (talk) 01:25, 18 July 2017 (UTC)
 * If you scan the talk above this section, you see no consensus whatsoever that "Arago spot" is a commonly used term, and three editors asking why the commonly used term, Poisson's spot, is not used. So I think your statement is backwards: if "Arago spot" is to be asserted as the common name, you need a reliable source saying so. Geoffrey.landis (talk) 14:45, 18 July 2017 (UTC)
 * We don't need consensus to keep the current article title; we need consensus to change it. The comments above are years old. Right now it's just you and me. Let's discuss and see if we can reach a consensus. Others will probably join the discussion.
 * It's not at all clear to me what the most common name for this phenomenon is. Can you provide quotes from the two sources you cited? The terms "Arago spot" and "Poisson's spot" are both common in books:
 * 
 * 
 * They are also both common in the scholarly literature:
 * 
 * 
 * Google counts are not really accurate, but the links establish common usage for both terms. You cited a dictionary of physics that uses Poisson's spot. Others refer to it as the Arago spot:
 * --Srleffler (talk) 01:41, 19 July 2017 (UTC)


 * "We don't need consensus to keep the current article title; we need consensus to change it."


 * Why do you say that?


 * "The comments above are years old."


 * Is that relevant? Why? Geoffrey.landis (talk) 17:19, 19 July 2017 (UTC)
 * The fundamental issue here is what the title of the article should be. Proposals to change the name of an article are resolved by determining whether there is a consensus among editors that the title should be changed. In the absence of such a consensus, the title stays as it is. This is a gross oversimplification; you can read all about renaming (moving) a page at Moving a page and Requested moves. Consensus reflects the current opinions of editors. Comments posted years ago will not be relevant in deciding on whether to rename a page.--Srleffler (talk) 05:25, 20 July 2017 (UTC)

Interesting list of links. I changed the search terms to "Arago's spot" OR "Arago spot" OR "Arago's bright spot" and "Poisson's spot" OR "Poisson spot" OR "Poisson's bright spot", which doesn't change the results much. "Arago's spot" appears more in books, "Poisson's spot" appears in scholarly articles. The scholarly article seach allows us to limit the search by date; limiting by date shows "Poisson's spot" (and variants) about 2.5 times more prevalant than "Arago's spot" (and variants) in post-2000 articles.

How do you suggest resolving the question? Geoffrey.landis (talk) 17:34, 19 July 2017 (UTC)
 * We should probably get more editors involved. One way of doing that would be for you to officially request that the page be renamed. You can do that by following the procedure at WP:RM. It's very easy.--Srleffler (talk) 05:25, 20 July 2017 (UTC)

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Math Questions
What are g and b in the integrals? ElizabethGreene (talk) 17:47, 23 April 2018 (UTC)


 * They are distances (g=P_0C and b=CP_1) in the diagram above the integrals (it would be better if they were marked on the diagram however, not just in the text...). L3erdnik (talk) 00:11, 24 April 2018 (UTC)

Claims Concerning Observation of the Spot Prior to Arago
Although there are a number of secondary claims of prior observation, I can vouch that the linked paper by Delisle contains no mention of the Arago spot. It speaks only of the rings surrounding a circular shadow. Here is a translation of the pertinent part (the remainder is a discussion of prior observations of eclipses and the experimental setup):

"I have noticed that in a great darkness, not only does a ring appear around the shadow of the opaque body which covers the Sun, but even that we see several others on the outside which are concentric to it, which are narrower, and whose light is weaker. It is easy to see three on paper in a dark room, but I have seen more than half a dozen of them by receiving on a scope the shadow of this body and putting the eye on the focus of the eyepiece. It is only the innermost of these rings that we see in broad daylight, the too great light of the air apparently making the others disappear, which are smaller and more weak. It might perhaps happen that if a solar eclipse darkens the air more, one could see more than one ring around the moon. When we see more rings, they are distinguished from each other not only by the strength of their light, which is becoming weaker as they move away from the shadow, but they are separated by small dark lines. The light of these rings is so bright that we see them in a dark room up to the image of the Sun; that is to say, that if one places the opaque body that covers the Sun so that it does not cover it entirely, and one receives on paper the shadow of this body, the part of the image of the Sun that is not covered by this body will also be present on this paper, and we will see up to this image, whose light is very bright, the light rings around the shadow, and the light of these rings will be even more vivid than that of the image of the Sun. The light of these rings is very white when they appear small, which is what happens when the plane on which they represent themselves is very close to the body; but as the plane is removed from the body, the rings appear larger, and the whiteness of each separates into the same colors into which refraction separates the white light of the sun, so that the whiteness of each of these rings is composed of the mixture of all these colors, which separate at a great distance, and form as many different series of colors as there are rings."

- M. Delisle le Cadet

The reference should be replaced by a correct one or the claim should be rescinded. -- Xerxes (talk) 15:14, 9 January 2019 (UTC)

Requested move 8 October 2020

 * The following is a closed discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. Editors desiring to contest the closing decision should consider a move review after discussing it on the closer's talk page. No further edits should be made to this discussion. 

The result of the move request was: Not moved - no consensus to move. (non-admin closure) --IWI (talk) 22:16, 25 October 2020 (UTC)

Arago spot → Poisson spot – (without the possessive) is the historically attested name for this phenomenon. it is i think uninformed to claim [see comments following] that "It is not established, however, that "Poisson's Spot" [sic] is the most common name for it." simply using google to crowd source the term "poisson spot" yields "about 10.9 million references", versus "arago spot" (~300,000 citations) or "arago's spot" (~400,000 mentions). and to Srleffler's tendentious comment that "google counts are not really accurate," does he assert that google counts have error bounds exceeding two orders of magnitude? a search on arxiv.org finds roughly four times as many cites with "poisson" as with "arago". the first citation (footnote) of the article refers to an authoritative source article titled, "Poisson spot." H.R. Suiter's "Star Testing Astronomical Telescopes" attests "This little bright patch is still called *Poisson's spot*." ... and so on. it is possible that there is a continental (french?) bias for one term over another, but in the anglophone literature there is only trivial dispute by any measure i can find.Drollere (talk) 21:59, 8 October 2020 (UTC) —Relisting. BegbertBiggs (talk) 16:00, 17 October 2020 (UTC)


 * Yes, Google "counts" are often off by orders of magnitude. See, for example, Google result counts are a meaningless metric. The numbers given by Google are not a count of anything at all, and are not accurate. Google establishes that these terms are all in common use, but cannot reliably tell us which term is the most common one for this particular phenomenon.--Srleffler (talk) 03:12, 9 October 2020 (UTC)
 * See also Search engine test--Srleffler (talk) 03:20, 9 October 2020 (UTC)
 * Oppose, for the moment. The nominator has not made an actual case for the change. He cites one not-particularly authoritative source, but one can easily cite many sources that use either "Poisson Spot" or "Arago Spot"/"Spot of Arago". Something more is required to establish which term is more common.--Srleffler (talk) 16:40, 10 October 2020 (UTC)


 * The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Basic requirements appear not to be sufficient
The basic requirements described (lower bound of Fresnel number and smoothness of the occluder) imply that one can observe Arago's spot arbitrarily close to the occluder. However, the numerical simulation diagram shows it absent for some distance behind the occluder (roughly for F > 10).

I think that either:
 * there's something missing in the basic requirements, or
 * the numerical simulation simulated a non-point source, which should then be mentioned (along with source's size) in the description of the diagram.

81.6.39.156 (talk) 10:53, 5 December 2021 (UTC)

Dubious image caption?


The images show simulated Arago spots in the shadow of a disc of varying diameter (4 mm, 2 mm, 1 mm – left to right) at a distance of 1 m from the disc. The point source has a wavelength of 633 nm (e.g. He-Ne Laser) and is located 1 m from the disc. The image width corresponds to 16 mm.

@Interferometrist tagged this image caption as dubious with the edit comment "Dubious 'image width corresponds to 16mm' - No looks like horiz. FOV just under twice diameter which =4mm. Or is incident light not collimated? Or did '4' mean Radius?" and the reason "looks like horiz. FOV just under twice diameter which =4mm."

The caption looks right to me. As described, the illumination is a point source not a collimated beam, and the 4 mm disc is located half way between the point source and the screen that was imaged. For the 4 mm disc, the image on the screen then ought to be about 8 mm across. Srleffler (talk) 21:49, 9 October 2022 (UTC)


 * Ok, that's right then. But then why not specify that the source beam was from a finite distance =1m? (Or did I MISS that??). More often someone would perform this using light from a laser + beam expander, thus collimated light, making the interpretation more obvious (shadow on same scale as obstructing object). Interferometrist (talk) 01:00, 10 October 2022 (UTC)
 * Oh! I see it now, on the graphic's page it indeed says that! But should also in the article. Interferometrist (talk) 01:02, 10 October 2022 (UTC)
 * Ok, I'm wrong again! It was correctly specified. I guess I need to learn to read before I write....
 * My sincere apologies to Srleffer and anyone else's time I wasted. Interferometrist (talk) 01:04, 10 October 2022 (UTC)

Questionable statement about "in astronomy"
I am a graduate physicist and have designed, built, owned, repaired, and used Newtonian telescopes for well over half a century now. So to me, this statement appears very wrong:

In astronomy, the Arago spot can also be observed in the strongly defocussed image of a star in a Newtonian telescope. There, the star provides an almost ideal point source at infinity, and the secondary mirror of the telescope constitutes the circular obstacle.

If you "strongly defocus" a Newtonian, you get a blurred image of the shadow / silhouette of the secondary, yes. And indeed, the edges of the mirror and the secondary holder are very heavily diffracted. BUT that image is nowhere near the size of the quoted Fresnel number. In something like even an f/11 teelscope, you are not going to get anywhere near the amount of diffracted light "bending" around the mirror to make a visible spot. So I dispute that statement. 2001:8003:E490:7D01:C00:4BC3:111:7AE7 (talk) 00:29, 1 September 2023 (UTC)