Lihat sumber bagi Serakan

{{Serakan}}
'''Serakan''' merupakan proses fizik am yang mana sesetengah bentuk [[sinaran]] seperti [[cahaya]], [[bunyi]], atau zarah bergerak, dipaksa untuk menyimpang dari [[trajektori]] yang lurus oleh satu atau lebih ketakseragaman yang tersetempat di dalam bahantara yang dilaluinya. Lazimnya, ini turut memaksudkan penyimpangan sinaran yang dipantulkan dari sudut menurut [[hukum pantulan]]. Pantulan yang terserak sering dipanggil ''[[pantulan resap]]'' manakala yang tak terserak dipanggil ''[[pantulan spekular]]'' (seakan cermin).

Terdapat pelbagai jenis bahan ketakseragaman yang boleh menyebabkan serakan, iaitu ''penyerak'' atau ''pusat serakan'', antaranya ialah [[zarah]], [[gelembung]], [[titisan]], naik turun [[ketumpatan]] [[bendalir]], [[hablur halus]] dalam pepejal [[polihablur]]an, kecacatan dalam pepejal [[monohablur]]an [[kekasaran permukaan]], [[Sel (biologi)|sel]] dalam organisme, dan [[gentian]] tekstil dalam pakaian. Kesan daripada ciri-ciri sebegini dalam trajektori hampir setiap jenis gelombang rambatan atau zarah bergerak boleh diterangkan dalam kerangka [[teori serakan]].

Antara bidang-bidang yang mana serakan dan teori serakan adalah penting ialah pengesanan radar, [[ultrabunyi perubatan]], pemeriksaan [[wafer semikonduktor]], pengawasan proses [[pempolimeran]], perjubinan akustik, komunikasi ruang bebas, dan [[imejan janaan komputer]].

== Serakan tunggal dan berganda ==
Sinaran yang diserakkan oleh satu penyerak sahaja dipanggil ''serakan tunggal'', tetapi selalunya penyerak-penyerak wujud secara berkelompok, maka sinaran ini boleh diserakkan banyak kali, iaitu ''serakan berganda''. Perbezaan utama antara kesan serakan tunggal dan berganda ialah, serakan tunggal biasanya boleh dianggap sebagai fenomena yang rawak, sementara serakan berganda pula lebih berketentuan pada kebiasaannya. Oleh sebab tidak jelas di mana letaknya sesuatu penyerak berbanding trajektori sinaran, maka hasil yang amat bergantung pada trajektori mendatang yang setepatnya kelihatan rawak kepada pemerhati. Satu contoh serakan tunggal ialah sebutir elektron yang dilepaskan ke arah nukleus atom, yang mana tidak diketahuinya kedudukan tepat atom tersebut berbanding dengan laluan elektron dan oleh itu sukar disukar, maka arah sebenar elektron itu selepas berlanggar pun tidak dipastikan, lebih-lebih lagi sifat mekanik kuantum turut merawakkan interaksi ini. Oleh itu, serakan tunggal sering diperikan oleh taburan kebarangkalian.

Bagi serakan berganda, kerawakan interaksi ini sering dirata-ratakan oleh jumlah kejadian serakan yang besar, oleh itu laluan akhir sinaran itu kelihatan sebagai taburan keamatan yang berketentuan. Contohnya, sebatang alur cahaya melalui [[kabus]] yang tebal. Serakan berganda amat serupa dengan [[resapan]], maka istilah ''serakan berganda'' dan ''resapan'' sering disamakan dalam konteks-konteks tertentu. Oleh yang demikian, unsur-unsur optik yang direka untuk menghasilkan serakan berganda dikenali sebagai ''peresap''. [[Serakan balik koheren]], iaitu peneguhan [[serakan balik]] yang berlaku apabila sinaran koheren diserakkan banyak kali oleh bahantara yang rawak, biasanya disebabkan oleh [[penyetempatan lemah]].

Walau bagaimanapun, bukan semua serakan tunggal adalah rawak. Sebagai contoh, alur laser yang dikawal dengan baik boleh diletakkan dengan tepat untuk menyerakkan zarah halus dengan hasil yang tentu. Situasi sebegini turut terdapat dalam serakan radar, yang mana sasarannya merupakan objek besar seperti manusia atau pesawat.

Begitu juga, serakan berganda adakalanya mendapat hasil yang agak rawak, terutama sekal dengan sinaran koheren. Turun naik yang rawak dalam keamatan sinaran koheren yang banyak kali terserak dipanggil ''[[bintik (fizik)|bintik]]''. Bintik juga berlaku jika satu gelombang koheren diserakkan oleh penyerak-penyerak yang berbeza. Jarang berlakunya serakan berganda yang hanya melibatkan sebilangan kecil interaksi yang mana kerawakannya tidak dirata-ratakan sepenuhnya. Sistem-sistem ini dianggap sebagai yang paling sukar untuk dimodelkan dengan tepat.
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The description of scattering and the distinction between single and multiple scattering are often highly involved with [[wave–particle duality]].

== Scattering theory ==
{{main|Scattering theory}}

''[[Scattering theory]]'' is a framework for studying and understanding the scattering of[[waves]] and [[Elementary particle|particles]].  Prosaically, wave scattering corresponds to the collision and scattering of a wave with some material object, for instance[[sunlight]] scattered by [[rain drop]]s to form a [[rainbow]]. Scattering also includes the interaction of [[billiard balls]] on a table, the [[Rutherford scattering]] (or angle change) of [[alpha particle]]s by [[gold]] [[atomic nucleus|nuclei]], the Bragg scattering (or diffraction) of electrons and X-rays by a cluster of atoms, and the inelastic scattering of a fission fragment as it traverses a thin foil. More precisely, scattering consists of the study of how solutions of [[partial differential equations]], propagating freely "in the distant past", come together and interact with one another or with a[[boundary condition]], and then propagate away "to the distant future".

== Electromagnetic scattering ==
[[Image:Electron-scattering.png|thumb|220px|A [[Feynman diagram]] of scattering between two electrons by emission of a virtual [[photon]].]]
[[Electromagnetic radiation|Electromagnetic waves]] are one of the best known and most commonly encountered forms of radiation that undergo scattering. [[light scattering|Scattering of light]] and radio waves (especially in [[radar]]) is particularly important. Several different aspects of electromagnetic scattering are distinct enough to have conventional names. Major forms of elastic light scattering (involving negligible energy transfer) are [[Rayleigh scattering]] and [[Mie theory|Mie scattering]]. Inelastic scattering includes [[Brillouin scattering]], [[Raman scattering]], inelastic [[X-ray]]scattering and [[Compton scattering]].

Light scattering is one of the two major physical processes that contribute to the visible appearance of most objects, the other being absorption. Surfaces described as ''white'' owe their appearance to multiple scattering of light by internal or surface inhomogeneities in the object, for example by the boundaries of transparent microscopic crystals that make up a stone or by the microscopic fibers in a sheet of paper. More generally, the [[Gloss (material appearance)|gloss]] (or [[Lustre (mineralogy)|lustre]] or [[Gloss (paint)|sheen]]) of the surface is determined by scattering. Highly scattering surfaces are described as being dull or having a matte finish, while the absence of surface scattering leads to a glossy appearance, as with polished metal or stone. Spectral absorption, the selective absorption of certain colors, determines the color of most objects with some modification by elastic scattering. The apparent blue color of [[veins]] in skin is a common example where both spectral absorption and scattering play important and complex roles in the coloration. Light scattering can also create color without absorption, often shades of blue, as with the sky ([[Rayleigh scattering]]), the human blue [[iris (anatomy)|iris]], and the feathers of some birds (Prum et al. 1998). However, resonant light scattering in [[nanoparticles]] can produce many different highly saturated and vibrant hues, especially when [[surface plasmon resonance]] is involved (Roqué et al. 2006).

Models of light scattering can be divided into three domains based on a dimensionless size parameter, α which is defined as
<math>
\mathit{\alpha}\mathrm{{=}}\mathrm{\frac{\mathit{\pi}\;{D_p}}{\mathit{\lambda}}}
</math>

where π''D<sub>p</sub>'' is the circumference of a particle and λ is the wavelength of incident radiation. Based on the value of α, these domains are:

<math>\alpha \ll 1</math>: [[Rayleigh scattering]] (small particle compared to wavelength of light)

<math>\alpha \approx 1</math>: [[Mie scattering]] (particle about the same size as wavelength of light)

<math>\alpha \gg 1</math>: Geometric scattering (particle much larger than wavelength of light)

[[Rayleigh scattering]] is a process in which electromagnetic radiation (including light) is scattered by a small spherical volume of variant refractive index, such as a particle, bubble, droplet, or even a density fluctuation. This effect was first modeled successfully by [[Lord Rayleigh]], from whom it gets its name. In order for Rayleigh's model to apply, the sphere must be much smaller in diameter than the [[wavelength]] (λ) of the scattered wave; typically the upper limit is taken to be about 1/10 the wavelength. In this size regime, the exact shape of the scattering center is usually not very significant and can often be treated as a sphere of equivalent volume. The inherent scattering that radiation undergoes passing through a pure gas is due to microscopic density fluctuations as the gas molecules move around, which are normally small enough in scale for Rayleigh's model to apply. This scattering mechanism is the primary cause of the blue color of the Earth's sky on a clear day, as the shorter blue wavelengths of sunlight passing overhead are more strongly scattered than the longer red wavelengths according to Rayleigh's famous 1/λ<sup>4</sup> relation. Along with absorption, such scattering is a major cause of the attenuation of radiation by the [[Earth's atmosphere|atmosphere]]. The degree of scattering varies as a function of the ratio of the particle diameter to the wavelength of the radiation, along with many other factors including [[Polarization (waves)|polarization]], angle, and [[Coherence (physics)|coherence]].

For larger diameters, the problem of electromagnetic scattering by spheres was first solved by [[Gustav Mie]], and scattering by spheres larger than the Rayleigh range is therefore usually known as [[Mie theory|Mie scattering]]. In the Mie regime, the shape of the scattering center becomes much more significant and the theory only applies well to spheres and, with some modification, [[spheroids]] and [[ellipsoids]]. Closed-form solutions for scattering by certain other simple shapes exist, but no general closed-form solution is known for arbitrary shapes.

Both Mie and Rayleigh scattering are considered elastic scattering processes, in which the energy (and thus wavelength and frequency) of the light is not substantially changed. However, electromagnetic radiation scattered by moving scattering centers does undergo a[[Doppler shift]], which can be detected and used to measure the velocity of the scattering center/s in forms of techniques such as [[LIDAR]] and [[radar]]. This shift involves a slight change in energy.

At values of the ratio of particle diameter to wavelength more than about 10, the laws of[[geometric optics]] are mostly sufficient to describe the interaction of light with the particle, and at this point the interaction is not usually described as scattering.

For modeling of scattering in cases where the Rayleigh and Mie models do not apply such as irregularly shaped particles, there are many numerical methods that can be used. The most common are [[finite element method|finite-element methods]] which solve [[Maxwell's equations]] to find the distribution of the scattered electromagnetic field. Sophisticated software packages exist which allow the user to specify the refractive index or indices of the scattering feature in space, creating a 2- or sometimes 3-dimensional model of the structure. For relatively large and complex structures, these models  usually require substantial execution times on a computer.
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== Rujukan ==
{{refbegin}}
*{{cite book |last= Bohren |first= Craig F.|author2=Donald R. Huffman  |title=Absorption and Scattering of Light by Small Particles |publisher= [[John Wiley & Sons|Wiley]] |year= 1983 |isbn= 978-0-471-29340-8 }}
*{{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 }}
*{{cite journal |last= Prum |first= Richard O.|author2=Rodolfo H. Torres |author2-link= Rodolfo H. Torres |author3=Scott Williamson |author4=Jan Dyck |year= 1998  |title= Coherent light scattering by blue feather barbs|journal=[[Nature]] |volume= 396 |issue= 6706|pages= 28–29  |doi= 10.1038/23838 |bibcode = 1998Natur.396...28P |s2cid= 4393904}}
*{{cite journal |last= Roqué|first= Josep  |author2=J. Molera |author3=P. Sciau |author4=E. Pantos |author5=M. Vendrell-Saz |year= 2006 |title= Copper and silver nanocrystals in lustre lead glazes: development and optical properties |journal=Journal of the European Ceramic Society |volume= 26|issue= 16|pages= 3813–3824 |doi= 10.1016/j.jeurceramsoc.2005.12.024 }}
* [[John H. Seinfeld|Seinfeld]], John H.; Pandis, Spyros N. (2006). Atmospheric Chemistry and Physics - From Air Pollution to Climate Change (2nd Ed.). John Wiley and Sons, Inc.ISBN 0-471-82857-2
*{{cite book |last= Stover |first= John C. |title= Optical Scattering: Measurement and Analysis |publisher= SPIE Optical Engineering Press |year= 1995 |isbn= 0-8194-1934-6 }}
{{refend}}

[[Category:Konsep fizik asas]]
[[Category:Serakan| ]]
[[Category:Fizik atom]]
[[Category:Fizik nuklear]]
[[Category:Fizik zarah]]