what is 散射??

Scattering

Scattering is a general physical process whereby some forms of radiation, such as light or moving particles, for example, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which it passes. It can also be seen with some sound phenomena. In conventional use, this also includes deviation of reflected radiation from the angle predicted by the law of reflection. Reflections that undergo scattering are often called diffuse reflections and unscattered reflections are called specular (mirror-like) reflections.

If the radiation is substantially or completely extinguished by the interaction (losing a significant proportion of its energy), the process is known as absorption. In some contexts, absorption is considered to be merely an extreme form of inelastic scattering. Generally speaking, in classical physics absorption and scattering tend to be treated as different phenomena, while in quantum physics absorption is treated as a form of scattering via the S-matrix. To be precise, absorption cannot occur without some degree of scattering, and scattering is rarely completely elastic, but on a macroscopic scale it is common for "absorption" and "scattering" to take place without any appreciable contribution from the other process, so treating them separately is often convenient.


With multiple scattering, the randomness of the interaction tends to be averaged out by the large number of scattering events, so that the final path of the radiation appears to be a deterministic distribution of intensity as the radiation is spread out. 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.

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 speckles. Speckle also occurs if multiple parts of a coherent wave scatter from different centers. In certain rare circumstances, multiple scattering may only involve 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 often highly involved with wave-particle duality.


Some areas where scattering and scattering theory are significant include radar sensing, medical ultrasound, semiconductor wafer inspection, polymerization process monitoring, acoustic tiling, free-space communications, and computer-generated imagery.





Contents[hide]

1 Electromagnetic scattering
2 Scattering in particle physics
3 See also
4 External links
5 References

圖片參考：http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/imgatm/rayle.gif










圖片參考：http://www.hk-phy.org/iq/skycolor/scattering.gif



圖片參考：http://www.hko.gov.hk/education/edu06nature/images/ele-sky-f1.gif



天空為什麼是藍的？






在天朗氣清的日子，天空一片蔚藍。要解釋這現象，我們首先要重溫陽光是由不同波長的光組成的–簡單地說是由不同顏色的光組成。

早於19世紀，英國科學家雷利勳爵研究光線遇上分子（包括空氣分子）時會出現改變方向的散射現象，並指出其散射程度與波長成反比。（嚴格來說，應是和波長四次方成反比）。



圖片參考：http://www.hko.gov.hk/education/edu06nature/images/ele-sky-f1.gif



換句話說，藍光（波長較短，約450納米）比紅光（波長較長，約600納米）更易被散射。這說明當我們望向天空的任何部份，但不直望太陽，我們看到的都是被散射出來的藍光。

這是太陽光被地球大氣散射 (scattering) 的結果。當陽光進入地球的大氣層後，空氣和水蒸氣的分子吸收部份陽光，再向四方八面輻射，這種現象稱為散射。白色的陽光是由不同顏色的光波合成的，以藍光波長最短，紅光波長最長，波長短的藍光較容易被散射。日落時夕陽接近地平線，陽光須穿過較厚的大氣層才到達地面 (圖一)，大部份藍光被散射，餘下紅光，所以夕陽呈現紅色（見附圖）。另一方面，由於白天時太陽光只穿過較薄的大氣層，藍光被散射的程度減少，所以太陽看起來是白色的，同時由於天空充滿了被散射的藍光，所以整個天空呈現藍色。