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# 4.0: Prelude to Diffraction

Figure $$\PageIndex{1}$$: A steel ball bearing illuminated by a laser does not cast a sharp, circular shadow. Instead, a series of diffraction fringes and a central bright spot are observed. Known as Poisson’s spot, the effect was first predicted by Augustin-Jean Fresnel (1788–1827) as a consequence of diffraction of light waves. Based on principles of ray optics, Siméon-Denis Poisson (1781–1840) argued against Fresnel’s prediction. (credit: modification of work by Harvard Natural Science Lecture Demonstrations)