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 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, Dominique-François-Jean Arago, decided to actually perform the experiment. 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 (later known as "Poisson's spot" or the "spot of Arago") had already been observed by Delisle and Maraldi 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), 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. Unlike attempts of the corpuscular theory to explain the propagation of light, the properties of photons have no role in explaining phenomena such as diffraction and interference; these are examples of light propagation strictly dependent on its wave character. Theory At the heart of Fresnel's wave theory is the Huygens–Fresnel principle, which states that every unobstructed point of a wavefront becomes the source of a secondary spherical wavelet and that the amplitude of the optical field E at a point on the screen is given by the superposition of all those secondary wavelets taking into account their relative phases. This