In optics, an Arago spot is a bright point which appears at the center of the shadow of a circular object in light from a point source. It is of considerable interest due to the historical part it played in the wave theory of light. In 1818 Siméon Poisson deduced from Augustin Fresnel's theory the necessity of a bright spot at the centre of the shadow of a circular opaque obstacle. With his counterintuitive result Poisson hoped to disprove the wave theory; however Dominique Arago experimentally verifed the prediction and today the demonstration goes by the name "Poisson's (or Arago's) spot." Since the spot occurs within the geometrical shadow, no particle theory of light could account for it, and its discovery in fact provided weighty evidence for the wave nature of light, much to Poisson's chagrin. Table of Opticks, 1728 Cyclopaedia Optics ( appearance or look in ancient Greek) is a branch of physics that describes the behavior and properties of light and the interaction of light with matter. ... Shadows on a pavement A shadow is a region of darkness where light is blocked. ... Hahahahahahahahahahahahhahahahahahahahahahahahahahahahahahahahahahhahaahahhhahhhahaahhhahahahahahahahahahahahahahahahahahahahahahahahahahahahhahahahahahaahhaaahahahahahahahahahahahahhahahahahahahahhahahahahahahahahahahahahahahahahhahahahahahhahahahhahahahhahahahahahhahahahhahahahhahahahhahahahahahahahahahhahahaahhaahahhaahhahaahahahahhahaahahsahahahahahshhsshshshagagaggagagagagagaggagagahahahahhahahahahahahahahahahahahhahahahahahahahahhahahahahahahahahahahyhahahahahhahaahahahahahahahahahahhahahahaah! your moma is a hoe. ... Look up point source in Wiktionary, the free dictionary. ... A wave is a disturbance that propagates through space or spacetime, often transferring energy. ... 1818 (MDCCCXVIII) is a common year starting on Thursday of the Gregorian calendar or a common year starting on Saturday of the 12-day slower Julian calendar. ... Siméon Poisson. ... Augustin Fresnel Augustin-Jean Fresnel (pronounced [] in AmE (or fray-NELL), [] in French) (May 10, 1788 – July 14, 1827), was a French physicist who contributed significantly to the establishment of the theory of wave optics. ... François Arago François Jean Dominique Arago (February 26, 1786 – October 2, 1853) was a French mathematician, physicist, astronomer, and politician. ... Siméon Poisson. ...

The existence of the spot had previously been observed in 1723 by Giacomo F. Maraldi, but the work had been largely unrecognized. Events February 16 - Louis XV of France attains his majority Births February 24 - John Burgoyne, British general (d. ... Giacomo Filippo Maraldi (August 21, 1665 – December 1, 1729) was an French-Italian astronomer and mathematician. ...

The presence of Arago's spot can be easily understood. When light shines on a circular obstacle, Huygens' principle says that every point along the circumference acts as a new point source of light. But clearly, the light coming from each of these points to the center of the shadow travels exactly the same distance, and so all the light arrives in phase and constructively interferes. But this argument holds inside the object's shadow, where geometric optics and particle theories of light predict there should be no light at all. Wave Refraction in the manner of Huygens. ... It has been suggested that this article or section be merged with Perimeter. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Interference of two circular waves - Wavelength (decreasing bottom to top) and Wave centers distance (increasing to the right). ... See also list of optical topics. ...

Notice also that the above argument works specifically for a circular obstacle. Many objects have a central bright peak in their diffraction pattern, but few of those display that peak inside their shadow. The ring pattern of a Fresnel zone plate is one such arrangement. Circle illustration In Euclidean geometry, a circle is the set of all points in a plane at a fixed distance, called the radius, from a fixed point, the centre. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... An example of a zone plate Zone plates are a device used to focus light. ...

The New Worlds Imager, a proposal to directly image extrasolar planets, relies on Arago's spot to isolate the reflected light of a planet from the sun it orbits. The New Worlds Imager is an NAIC funded project, headed by Dr. Webster Cash of the University of Colorado at Boulder. ...