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Arago point

  1. One of the three commonly detectable points, called neutral points, along the vertical circle through the Sun at which the degree of polarization of diffuse sky radiation goes to zero. The Arago point (also known as the Arago spot) typically lies about 20 above the antisolar point (where an imaginary ray connecting the Sun and the observer meets the sky), but is found at higher altitudes in turbid air. The latter property makes the Arago distance a useful measure of atmospheric turbidity. The other two neutral points are called the Babinet point (15 to 20° directly above the Sun, hence difficult to detect because of solar glare) and the Brewster point (15 to 20° directly below the Sun).

  2. In optics, a bright spot that appears in the center of the shadow of a circular disk illuminated by a point source, caused by diffraction.