Arago spot
Noun
Meaning
Arago spot (plural Arago spots)
(optics) A bright point that appears at the centre of a circular object's shadow due to Fresnel diffraction.
Synonyms: Fresnel spot, Poisson spot
The Arago 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.
Examples
Arago spot with matter waves Recently, the Arago spot experiment was demonstrated with a supersonic expansion beam of deuterium molecules (an example of neutral matter waves ).
An Arago spot of electrons, which also constitute matter waves, can be observed in transmission electron microscopes when examining circular structures of a certain size.
Arago later noted that the diffraction bright spot (which later became known as both the Arago spot and the Poisson spot) had already been observed by Joseph-Nicolas Delisle and Giacomo F. Maraldi citation a century earlier.
Deviation from circularity If the cross-section of the circular object deviates slightly from its circular shape (but it still has a sharp edge on a smaller scale) the shape of the point-source Arago spot changes.
Finite source size and spatial coherence The main reason why the Arago spot is hard to observe in circular shadows from conventional light sources is that such light sources are bad approximations of point sources.
Furthermore, this means that the Arago spot is present even just a few obstacle diameters behind the disc.