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How does diffraction cause light to spread into a geometric shadow?

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In the context of optics, diffraction causes light to spread into a geometric shadow because of the wave nature of light and the principle of interference.12

According to Huygens’s principle, every point on a wave front acts as a secondary source of spherical wavelets. When a light wave encounters an obstacle or an aperture, the portions of the wave that are not blocked continue to propagate. These secondary wavelets overlap and interfere with one another in the region beyond the obstruction.2

Because these waves superimpose, they can reinforce or cancel each other out, creating a pattern of light and dark fringes within the area that would otherwise be a sharp geometric shadow. This bending of light is most pronounced when the size of the obstacle or aperture is comparable to the wavelength of the light. Since visible light has a very short wavelength, this spreading is typically subtle and requires precise measurement to observe.12