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Rayleigh scattering (pronounced ), named after the British physicist Lord Rayleigh (John William Strutt),〔Lord Rayleigh (John Strutt) refined his theory of scattering in a series of papers that were issued over a period of decades. Here is a partial list of those papers: # John Strutt (1871) "On the light from the sky, its polarization and colour," ''Philosophical Magazine'', series 4, vol.41, pages 107-120, 274-279. # John Strutt (1871) "On the scattering of light by small particles," ''Philosophical Magazine'', series 4, vol. 41, pages 447-454. # John Strutt (1881) "On the electromagnetic theory of light," ''Philosophical Magazine'', series 5, vol. 12, pages 81-101. # John Strutt (1899) "On the transmission of light through an atmosphere containing small particles in suspension, and on the origin of the blue of the sky," ''Philosophical Magazine'', series 5, vol. 47, pages 375-394. 〕 is the (dominantly) elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the radiation. Rayleigh scattering does not change the state of material, hence it is a parametric process. The particles may be individual atoms or molecules. It can occur when light travels through transparent solids and liquids, but is most prominently seen in gases. Rayleigh scattering results from the electric polarizability of the particles. The oscillating electric field of a light wave acts on the charges within a particle, causing them to move at the same frequency. The particle therefore becomes a small radiating dipole whose radiation we see as scattered light. Rayleigh scattering of sunlight in the atmosphere causes diffuse sky radiation, which is the reason for the blue color of the sky and the yellow tone of the sun itself. Rayleigh scattering of molecular nitrogen and oxygen in the atmosphere includes elastic scattering as well as the inelastic contribution from rotational Raman scattering in air, since the changes in wavenumber of the scattered photon are typically smaller than 50 cm−1.〔A. T. Young, "Rayleigh scattering," Appl. Opt. 20, 522–535, 1981〕 This can lead to changes in the rotational state of the molecules. Furthermore, the inelastic contribution has the same wavelengths dependency as the elastic part. Scattering by particles similar to, or larger than, the wavelength of light is typically treated by the Mie theory, the discrete dipole approximation and other computational techniques. Rayleigh scattering applies to particles that are small with respect to wavelengths of light, and that are optically "soft" (i.e. with a refractive index close to 1). On the other hand, Anomalous Diffraction Theory applies to optically soft but larger particles. ==Small size parameter approximation== The size of a scattering particle is often parameterized by the ratio , where ''r'' is its characteristic length (radius) and ''λ'' is the wavelength of the light. The amplitude of light scattered from within any transparent dielectric is proportional to the inverse square of its wavelength and to the volume of material, that is to the cube of its characteristic length. The wavelength dependence is characteristic of dipole scattering〔 and the volume dependence will apply to any scattering mechanism. Objects with x ≫ 1 act as geometric shapes, scattering light according to their projected area. At the intermediate x ≃ 1 of Mie scattering, interference effects develop through phase variations over the object's surface. Rayleigh scattering applies to the case when the scattering particle is very small (x ≪ 1, with a particle size < 1 /10 wavelength〔http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html〕) and the whole surface re-radiates with the same phase. Because the particles are randomly positioned, the scattered light arrives at a particular point with a random collection of phases; it is incoherent and the resulting intensity is just the sum of the squares of the amplitudes from each particle and therefore proportional to the inverse fourth power of the wavelength and the sixth power of its size. In detail, the intensity ''I'' of light scattered by any one of the small spheres of diameter ''d'' and refractive index n from a beam of unpolarized light of wavelength ''λ'' and intensity ''I''0 is given by :,〔Seinfeld and Pandis, ''Atmospheric Chemistry and Physics, 2nd Edition'', John Wiley and Sons, New Jersey 2006, Chapter 15.1.1〕 where ''R'' is the distance to the particle and ''θ'' is the scattering angle. Averaging this over all angles gives the Rayleigh scattering cross-section :.〔Siegel, R., Howell, J.R., (2002). Thermal radiation heat transfer. p. 480. New York, NY: Taylor & Francis.〕 The fraction of light scattered by a group of scattering particles is the number of particles per unit volume ''N'' times the cross-section. For example, the major constituent of the atmosphere, nitrogen, has a Rayleigh cross section of at a wavelength of 532 nm (green light). This means that at atmospheric pressure, where there are about molecules per cubic meter, about a fraction 10−5 of the light will be scattered for every meter of travel. The strong wavelength dependence of the scattering (~''λ''−4) means that shorter (blue) wavelengths are scattered more strongly than longer (red) wavelengths. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Rayleigh scattering」の詳細全文を読む スポンサード リンク
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