Editing Scattering (section)

==Single and multiple scattering==
[[File:Zodiacal Glow Lightens Paranal Sky.jpg|thumb|[[Zodiacal light]] is a faint, diffuse glow visible in the [[night sky]]. The phenomenon stems from the [[light scattering by particles|scattering]] of [[sunlight]] by [[interplanetary dust cloud|interplanetary dust]] spread throughout [[invariable plane|the plane]] of the [[Solar System]].<ref>{{cite news |url=http://www.eso.org/public/images/potw1348a/ |title=Zodiacal Glow Lightens Paranal Sky |work=ESO Picture of the Week |publisher=[[European Southern Observatory]] |access-date=2 December 2013}}</ref>]]
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When radiation is only scattered by one localized scattering center, this is called ''single scattering''. It is more common that scattering centers are grouped together; in such cases, radiation may scatter many times, in what is known as ''multiple scattering''.<ref>{{cite book |last= Gonis |first= Antonios |author2=William H. Butler |title= Multiple Scattering in Solids |publisher= [[Springer Science+Business Media|Springer]] |year= 1999 |isbn= 978-0-387-98853-5 }}</ref> The main difference between the effects of single and multiple scattering is that single scattering can usually be treated as a random phenomenon, whereas multiple scattering, somewhat counterintuitively, can be modeled as a more deterministic process because the combined results of a large number of scattering events tend to average out. Multiple scattering can thus often be modeled well with [[diffusion theory]].<ref>{{cite book |last= Gonis |first= Antonios |author2=William H. Butler |title= Multiple Scattering in Solids |publisher= [[Springer Science+Business Media|Springer]] |year= 1999 |isbn= 978-0-387-98853-5 }}</ref>

Because the location of a single scattering center is not usually well known relative to the path of the radiation, the outcome, which tends to depend strongly on the exact incoming trajectory, appears random to an observer. This type of scattering would be exemplified by an electron being fired at an atomic nucleus. In this case, the atom's exact position relative to the path of the electron is unknown and would be unmeasurable, so the exact trajectory of the electron after the collision cannot be predicted. Single scattering is therefore often described by probability distributions.

With multiple scattering, the randomness of the interaction tends to be averaged out by a large number of scattering events, so that the final path of the radiation appears to be a deterministic distribution of intensity. This is exemplified by a [[light beam]] passing through thick [[fog]]. Multiple scattering is highly analogous to [[diffusion]], and the terms ''multiple scattering'' and ''diffusion'' are interchangeable in many contexts. Optical elements designed to produce multiple scattering are thus known as ''diffusers''.<ref>{{cite book |last= Stover |first= John C. |title= Optical Scattering: Measurement and Analysis |publisher= SPIE Optical Engineering Press |year= 1995 |isbn= 978-0-8194-1934-7 }}</ref> [[Coherent backscattering]], an enhancement of [[backscattering]] that occurs when coherent radiation is multiply scattered by a random medium, is usually attributed to [[weak localization]].

Not all single scattering is random, however. A well-controlled laser beam can be exactly positioned to scatter off a microscopic particle with a deterministic outcome, for instance. Such situations are encountered in [[radar]] scattering as well, where the targets tend to be macroscopic objects such as people or aircraft.

Similarly, multiple scattering can sometimes have somewhat random outcomes, particularly with coherent radiation. The random fluctuations in the multiply scattered intensity of coherent radiation are called [[speckle pattern|speckle]]s. Speckle also occurs if multiple parts of a coherent wave scatter from different centers. In certain rare circumstances, multiple scattering may only involve a small number of interactions such that the randomness is not completely averaged out. These systems are considered to be some of the most difficult to model accurately.

The description of scattering and the distinction between single and multiple scattering are tightly related to [[wave–particle duality]].