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The divergence of light from its initial line of travel is called diffraction. In general, diffraction occurs when waves pass through small openings, around obstacles, or past sharp edges, as shown in figure. When an opaque object is placed between a point source of light and a screen, no sharp boundary exists on the screen between a shadowed region and an illuminated region. The illuminated region above the shadow of the object contains alternating light and dark fringes. Such a display is called a diffraction pattern.
Diffraction supports the wave theory of light. After 1860, the rapid development of spectroscopy (the study of radiation by dispersing its component frequencies and attaching a relative intensity to each) and refinements in diffraction gratings increased the validity of the wave theory of light. In keeping with the rise of an electromagnetic, rather than strictly mechanical, ontology characteristic of the later nineteenth century, Maxwell's work explained light waves as oscillations in the equilibrium configuration of electric and magnetic fields, and he derived a wave equation for the spread of electromagnetic effects, the speed of whose propagation came remarkably close, he observed, to that of light. Decisive confirmation of the wave theory of light would wait until 1887/88, however, when Heinrich Hertz demonstrated that Maxwell's waves could be reflected, refracted, and polarized (just like light) and that they travel at light's velocity.
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Waves and Optics