Editing Forward scatter

{{Short description|Small angle deflection of waves}}
[[File:Saturn eclipse.jpg|thumb|Saturn [[eclipse]]s the Sun, as seen from the ''[[Cassini–Huygens|Cassini]]'' [[space probe]]. The forward [[light scattering by particles|scattering of light]] makes the faint outer rings more visible.]]'''Forward scattering''' is the [[diffraction|deflection]] of [[wave]]s by small angles so that they continue to move in close to the same direction as before the scattering. It can occur with all types of waves, for instance [[light]], [[ultraviolet]] radiation, [[X-ray]]s as well as [[matter wave]]s such as [[electron]]s, [[neutron]]s and even [[water waves]]. It can be due to [[diffraction]], [[refraction]], and low angle [[reflection (physics)|reflection]]. It almost always occurs when the [[wavelength]] of the radiation used is small relative to the features which lead to the scattering. '''Forward scatter''' is essentially the reverse of [[backscatter]].

Many different examples exist, and there are very large fields where forward scattering dominates, in particular for [[electron diffraction]] and [[Electron microscope|electron microscopy]], [[X-ray diffraction]] and [[neutron diffraction]]. In these the relevant waves are transmitted through the samples. One case where there is forward scattering in a reflection geometry is [[reflection high-energy electron diffraction]].
[[File:Rixs_cartoon.png|thumb|Forward scattering of a photon, with possible energy change.]]

== General description ==
Whenever waves encounter obstacles of any type there are changes in the direction of the waves ([[wave vector]]) by diffraction,<ref name=":0">{{Cite book |last=Cowley |first=J. M. |title=Diffraction physics |date=1995 |publisher=Elsevier Science B.V |isbn=978-0-444-82218-5 |edition=3rd |series=North-Holland personal library |location=Amsterdam ; New York}}</ref><ref>{{Cite book |last=Born |first=Max |title=[[Principles of optics]]: electromagnetic theory of propagation, interference and diffraction of light |last2=Wolf |first2=Emil |date=2017 |publisher=Cambridge University Press |others=Avadh B. Bhatia |isbn=978-0-521-64222-4 |edition=Seventh (expanded) edition, 13th printing |location=Cambridge}}</ref> and sometimes its energy by [[inelastic scattering]]. These processes occur for all types of waves, although how they behave varies with both their type and that of the obstacle. As illustrated in the figure, if the change in the wave vector '''q''' is fairly small the scattered wave moves in close to the same direction as the input—it has been scattered. In most cases the change in the wave vector scales inversely with the size of obstacles, so forward scattering is more common when the obstacles are large compared to the [[wavelength]] of the radiation.
[[File:Wave_diffraction_at_the_Blue_Lagoon,_Abereiddy.jpg|thumb|Wave diffraction at the Blue Lagoon, Abereiddy]]
In many cases the waves of interest have relatively small wavelengths, for instance high-energy electrons<ref name=":0" /> or [[X-ray]]s.<ref>{{Cite book |last=Warren |first=B. E. |url=https://www.google.com/books/edition/X_ray_Diffraction/wfLBhAbEYAsC?hl=en |title=X-ray diffraction |date=1990 |publisher=Dover Publications |isbn=978-0-486-66317-3 |edition= |location=New York}}</ref> However, the process is very general and can also be seen when water flows through a narrow channel as shown in the figure at the Blue Lagoon.

== Comets ==
Forward scattering can make a back-lit comet appear significantly brighter because the dust and [[ice crystal]]s are reflecting and enhancing the apparent brightness of the comet by scattering that light towards the observer.<ref name="scattering">{{cite web
  |title=Comet Elenin as seen by STEREO-B, and what we think is going to happen next...
  |publisher=Sungrazing Comets
  |url=https://sungrazer.nrl.navy.mil/index.php?p=Elenin
  |access-date=2011-08-05
  |archive-date=2011-09-26
  |archive-url=https://web.archive.org/web/20110926203659/http://sungrazer.nrl.navy.mil/index.php?p=Elenin
  |url-status=live
  }}</ref> Comets studied forward-scattering in visible-thermal photometry include [[Comet Skjellerup–Maristany|C/1927 X1 (Skjellerup–Maristany)]], [[Comet West|C/1975 V1 (West)]], and C/1980 Y1 (Bradfield).<ref name=Marcus2007>{{cite journal
  |last=Marcus |first=Joseph C.
  |title=Forward-Scattering Enhancement of Comet Brightness. I. Background and Model
  |journal=International Comet Quarterly
  |volume=29 |issue=2 |pages=39–66 |year=2007
  |bibcode=2007ICQ....29...39M}}</ref> Comets studied forward-scattering in [[Solar and Heliospheric Observatory|SOHO]] non-thermal C3 [[coronograph]] photometry include [[96P/Machholz]] and C/2004 F4 (Bradfield).<ref name=Marcus2007/> The brightness of the [[great comet]]s [[C/2006 P1]] (McNaught) and [[Comet Skjellerup–Maristany]] near [[perihelion]] were enhanced by forward scattering.<ref name="fscat">{{Cite journal
 |url         = http://www.icq.eps.harvard.edu/marcus_icq29_119.pdf
 |bibcode     = 2007ICQ....29..119M
 |title       = Forward-Scattering Enhancement of Comet Brightness. II. The Light Curve of C/2006 P1
 |journal     = International Comet Quarterly
 |volume      = 29
 |first       = Joseph N.
 |last        = Marcus
 |date        = October 2007
 |pages       = 119–130
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20110708051321/http://www.icq.eps.harvard.edu/marcus_icq29_119.pdf
 |archive-date = 2011-07-08
}}</ref>

== See also ==

* {{Annotated link|Back scattering}}
* {{annotated link|Elastic scattering}}
* {{Annotated link|Inelastic scattering}}
* {{annotated link|Scattering}}

==References==
{{Reflist}}

== External links ==
*[https://web.archive.org/web/20120319150521/http://www2.jpl.nasa.gov/basics/saturn/ Cassini's Views of Saturn's Rings]
*[http://spacecraftkits.com/scatter/ Light Scattering Demonstration]

[[Category:Radio frequency propagation]]
[[Category:Scattering, absorption and radiative transfer (optics)]]