![]() ![]() ) This was one of the investigations that led to the victory of the wave theory of light over then predominant corpuscular theory. (Lisle had observed this fifty years earlier. However, Arago, another member of the committee, performed the experiment and showed that the prediction was correct. He used Fresnel's theory to predict that a bright spot ought to appear in the center of the shadow of a small disc, and deduced from this that the theory was incorrect. Poisson was a member of the French Academy, which reviewed Fresnel's work. These assumptions have no obvious physical foundation but led to predictions that agreed with many experimental observations, including the Poisson spot. To obtain agreement with experimental results, he had to include additional arbitrary assumptions about the phase and amplitude of the secondary waves, and also an obliquity factor. ![]() In 1818, Fresnel showed that Huygens's principle, together with his own principle of interference could explain both the rectilinear propagation of light and also diffraction effects. The resolution is that the source is a dipole (not the monopole assumed by Huygens), which cancels in the reflected direction. The resolution of this error was finally explained by David A. He was able to provide a qualitative explanation of linear and spherical wave propagation, and to derive the laws of reflection and refraction using this principle, but could not explain the deviations from rectilinear propagation that occur when light encounters edges, apertures and screens, commonly known as diffraction effects. He assumed that the secondary waves travelled only in the "forward" direction and it is not explained in the theory why this is the case. In 1678, Huygens proposed that every point to which a luminous disturbance reaches becomes a source of a spherical wave the sum of these secondary waves determines the form of the wave at any subsequent time. Diffraction of a plane wave when the slit width equals the wavelength
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