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In optics, an Arago spot, Fresnel bright spot, or Poisson spot is a bright point that appears at the center of a circular object's shadow due to Fresnel diffraction.〔〔〔〔 This spot played an important role in the discovery of the wave nature of light and is a common way to demonstrate that light behaves as a wave (for example, in undergraduate physics laboratory exercises). The basic experimental setup requires a "point source," such as an illuminated pinhole, or a diverging laser beam. The dimensions of the setup must comply with the requirements for Fresnel diffraction. Namely, the Fresnel number must satisfy : where : ''d'' is the diameter of the circular object : ''ℓ'' is the distance between the object and the screen : ''λ'' the wavelength of the source Finally, the edge of the circular object must be sufficiently smooth. These conditions together explain why the bright spot is not encountered in everyday life. However, with the laser sources available today, it is undemanding to perform an Arago spot experiment.〔http://www.princeton.edu/~rvdb/images/Questar/PoissonSpot.html〕 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. When light shines on the circular obstacle, Huygens' principle says that every point in the plane of the obstacle acts as a new point source of light. The light coming from points on the circumference of the obstacle, and going to the center of the shadow, travels exactly the same distance; so all the light passing close by the object arrives at the screen in phase and constructively interferes. This results in a bright spot at the shadow's center, where geometrical optics and particle theories of light predict that there should be no light at all. ==History== At the beginning of the 19th century, the idea that light does not simply propagate along straight lines gained traction. Thomas Young published his double-slit experiment in 1807.〔 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.〔 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.〔 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 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. Since the Arago spot is not easily observed in every-day situations, Poisson interpreted it as an absurd result and that it should disprove Fresnel's theory. However, the head of the committee, Dominique-François-Jean Arago—who incidentally later became Prime Minister of France—decided to perform the experiment in more detail. He molded a 2 mm metallic disk to a glass plate with wax.〔 He succeeded in observing the predicted spot, which convinced most scientists of the wave-nature of light, and gave Fresnel the win. Arago later noted that the phenomenon (which was later to be known as Poisson's Spot or the Spot of Arago) had already been observed by Delisle〔 and Maraldi〔 a century earlier. It only turned out much later (in one of Einstein's ''Annus Mirabilis'' papers) that light can be equally described as a particle (wave–particle duality of light). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Arago spot」の詳細全文を読む スポンサード リンク
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