When an electromagnetic wave interacts with a dense random medium, the amount of energy scattered in the retroreflection direction (namely, in that of back-scattering), is remarkably larger, on the average, than in any other direction. Examples of optical phenomena resulting from this effect have been known for a long time: the optical glory formed by the bright colored rings surrounding the shadow of an illuminated object over a region of mist, (e.g., the shadow of an airplane over the clouds),
is due to a large retroreflectance of light by Mie scattering from the water droplets. Similarly, the heiligenschein consists of a bright region of backscattered light about the shadow that an illuminated object projects over dew-covered grass. These phenomena belong to a wider class of so-called opposition effects; also described in astronomical observations, and among which higher reflectivities of the moon when it is full have been noticed since 1922. More recently, enhanced backscattering of light as it propagates in dense media has been reported.
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