GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY
Arago spot, Photo 2 of 2..A steel ball is placed in a beam produced by either a Helium-Neon laser operating at 632.8 nm (red, Photo 1) or a Helium-Neon laser operating at 543.4 nm (green, Photo 2). The resulting diffraction patterns are observed at the same distance from the ball. Note that the fringe spacing is smaller for the green laser. Also note the bright spot, called Arago or Poisson spot, at the center of the geometric shadow. Historically, observation of Arago spot was used as an evidence for the wave theory of light.
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