Editing Arago spot (section)

== History ==

At the beginning of the 19th century, the idea that light does not simply propagate along straight lines gained traction. [[Thomas Young (scientist)|Thomas Young]] published his [[Young's double-slit interferometer|double-slit experiment]] in 1807.<ref name="young1807"/> The original Arago spot experiment was carried out a decade later and was the deciding experiment on the question of whether light is a particle or a wave. It is thus an example of an ''[[experimentum crucis]]''.

At that time, many favored Isaac Newton's corpuscular theory of light, among them the theoretician [[Siméon Denis Poisson]].<ref name="newton1704"/> In 1818 the [[French Academy of Sciences]] launched a competition to explain the properties of light, where Poisson was one of the members of the judging committee. The civil engineer [[Augustin-Jean Fresnel]] entered this competition by submitting a new [[wave theory of light]].<ref name="fresnel1868"/>

Poisson studied Fresnel's theory in detail and, being a supporter of the particle theory of light, looked for a way to prove it wrong. Poisson thought that he had found a flaw when he argued that a consequence of Fresnel's theory was that there would exist an on-axis bright spot in the shadow of a circular obstacle, where there should be complete darkness according to the particle theory of light. This prediction was seen as an absurd consequence of the wave theory, and the failure of that prediction should be a strong argument to reject Fresnel's theory.

However, the head of the committee, [[François Arago|Dominique-François-Jean Arago]], decided to actually perform the experiment. He molded a 2&nbsp;mm metallic disk to a glass plate with wax.<ref name="fresnel1868_arago"/> He succeeded in observing the predicted spot, confirming Fresnel's prediction.<ref name="bornwolf"/>{{rp|375}}<ref>{{cite journal|last1=Arago|title=Rapport fait par M. Arago à l'Académie des Sciences, au nom de la Commission qui avait été chargée d'examiner les Mémoires envoyés au concours pour le prix de la diffraction.|journal=Annales de Chimie et de Physique|date=1819|volume=11|pages=5–30|url=https://babel.hathitrust.org/cgi/pt?id=iau.31858046217711;view=1up;seq=11|series=2nd series|trans-title=Report made by Mr. Arago to the Academy of Sciences in the name of the commission which had been charged with examining the memoirs submitted to the competition for the diffraction prize.|language=fr}} [https://babel.hathitrust.org/cgi/pt?id=iau.31858046217711;view=1up;seq=22 From p. 16:]  ''"L'un de vos commissaires, M. Poisson, avait déduit des intégrales rapportées par l'auteur, le résultat singulier que le centre de l'ombre d'un écran circulaire opaque devait, lorsque les rayons y pénétraient sous des incidences peu obliques, être aussi éclairé que si l'écran n'existait pas.  Cette conséquence a été soumise à l'épreuve d'une expérience directe, et l'observation a parfaitement confirmé le calcul (e)."''  (One of your commissioners, Mr. Poisson, had deduced from the integrals [that had been] reported by the author [i.e., Mr. Fresnel], the strange result that the center of the shadow of an opaque circular screen should — when the [light] rays penetrate it [i.e., the shadow] at slightly oblique incidences — also be illuminated as if the screen didn't exist.  This result has been submitted to the test of a direct experiment, and observation has perfectly confirmed the calculation (e).)</ref>

Arago later noted<ref>{{cite book |last=Arago |first=F. |chapter=Mémoire sur la méthode des interférences appliquée à la recherche des indices de réfraction. |title=Œuvres complètes |pages=312–334 |quote=Lorsqu'un corps opaque est placé dans un faisceau de lumière, son ombre est bordée à l'extérieur de bandes de diverses nuances et de diverses largeurs. Ces bandes ont été étudiées par Newton dans le premier livre de son Optique; mais ce célèbre physicien ne parle pas des bandes non moins remarquables qui se forment dans l'intérieur de l'ombre des corps déliés, quoique Grimaldi en eût déjà donné une description détaillée dans son ouvrage, et il affirme même positivement qu'aucune lumière ne pénètre dans l'ombre géométrique. L'inexactitude de ce résultat fut suffisamment prouvée par Maraldi et De l'Isle, qui, du reste, n'ajoutèrent rien de saillant à ce que Grimaldi avait découvert longtemps avant. |trans-quote=When an opaque body is placed in a beam of light, its shadow is bordered on the outside by bands of various shades and widths. These bands were studied by Newton in the first book of his Optics; but this famous physicist does not speak of the no less remarkable bands which form in the interior of the shadow of loose bodies, although Grimaldi had already given a detailed description of them in his work, and he even affirms positively that no light enters the geometric shadow. The inaccuracy of this result was sufficiently proven by Maraldi and De l'Isle, who, moreover, added nothing salient to what Grimaldi had discovered long before.}}</ref> that the phenomenon (later known as "Poisson's spot" or the "spot of Arago") had already been observed by [[Joseph-Nicolas Delisle|Delisle]]<ref name="delisle1715"/> and [[Giacomo F. Maraldi|Maraldi]]<ref name="maraldi1723"/> a century earlier.

Although Arago's experimental result was overwhelming evidence in favor of the wave theory, a century later, in conjunction with the birth of [[quantum mechanics]] (and first suggested in one of [[Albert Einstein]]'s [[Annus Mirabilis papers|''Annus Mirabilis'' papers]]), it became understood that light (as well as all forms of matter and energy) must be described as both a particle and a wave ([[wave–particle duality]]). However the particle associated with electromagnetic waves, the [[photon]], has nothing in common with the particles imagined in the corpuscular theory that had been dominant before the rise of the wave theory and Arago's powerful demonstration. Before the advent of quantum theory in the late 1920s, only the wave nature of light could explain phenomena such as [[diffraction]] and [[Wave interference|interference]]. Today it is known that a diffraction pattern appears through the mosaic-like buildup of bright spots caused by single photons, as predicted by Dirac's quantum theory. With increasing light intensity the bright dots in the mosaic diffraction pattern just assemble faster. In contrast, the wave theory predicts the formation of an extended continuous pattern whose overall brightness increases with light intensity.