- 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).
- 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.