Arago Spot - History

History

The original Arago spot experiment was carried out in the beginning of the 19th century and played an important role in the history of science. Then it turned out to be the deciding experiment of whether light is a particle or a wave. It is thus a great example of a so-called experimentum crucis. 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).

At the beginning of the 19th century it became more and more evident that light does not simply propagate along straight lines (Thomas Young published his double-slit experiment in 1807 ). However, many still favored Isaac Newton's corpuscular theory of light, among them the great theoretician Siméon-Denis Poisson. In 1818 the French Academy of Sciences launched therefore 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 of course looked for a way to prove it wrong being a supporter of the particle-theory of light. 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. As mentioned before the Arago spot is not easily observed in every-day situations, so it was only natural for Poisson to interpret it as an absurd result and that it should disprove Fresnel's theory.

However, the head of the committee, Dominique-François-Jean Arago, and 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. To everyone's surprise he succeeded in observing the predicted spot, which convinced most scientists of the wave-nature of light. In the end Fresnel won the competition, much to Poisson's chagrin. 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.