http://en.wikipedia.org/wiki/Rayleigh_scattering
John William Strutt, 3rd Baron Rayleigh, OM, PRS (/ˈreɪli/; 12 November 1842 – 30 June 1919) was an English physicist who, with William Ramsay, discovered argon, an achievement for which he earned the Nobel Prize for Physics in 1904. He also discovered the phenomenon now called Rayleigh scattering, explaining why the sky is blue, and predicted the existence of the surface waves now known as Rayleigh waves. Rayleigh's textbook, The Theory of Sound, is still referred to by acoustic engineers today.
Rayleigh scattering causes the blue hue of the daytime sky and the reddening of the sun at sunset.
Rayleigh scattering is more evident after sunset. This picture was taken about one hour after sunset at 500m altitude, looking at the horizon where the sun had set.
Rayleigh scattering (pronounced /ˈreɪli/ RAY-lee), named after the British physicist Lord Rayleigh,[1] is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. After the Rayleigh scattering the state of material remains unchanged, hence Rayleigh scattering is also said to be 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.
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.
The intensity I of light scattered by a single small particle from a beam of unpolarized light of wavelength λ and intensity I0 is given by:
where R is the distance to the particle, θ is the scattering angle, n is the refractive index of the particle, and d is the diameter of the particle. The Rayleigh scattering cross-section is given by [4]
Reason for the blue color of the sky
Further information: Diffuse sky radiation
Scattered blue light is polarized. The picture on the right is shot through a polarizing filter which removes light that is linearly polarized in a specific direction.
A portion of the beam of light coming from the sun scatters off molecules of gas and other small particles in the atmosphere. Here, Rayleigh scattering primarily occurs through sunlight's interaction with randomly located air molecules. Exactly equivalently, but from a purely macroscopic point of view, the scattering comes from the microscopic density fluctuations which result from the random distribution of molecules in the air. A region of higher or lower density has a slightly different refractive index from the surrounding medium, and therefore it acts like a short-lived scattering particle. It is this scattered light that gives the surrounding sky its brightness and its color. As previously stated, Rayleigh scattering is inversely proportional to the fourth power of wavelength, so that shorter wavelength violet and blue light will scatter more than the longer wavelengths (yellow and especially red light). However, the Sun, like any star, has its own spectrum and so I0 in the scattering formula above is not constant but falls away in the violet. In addition the oxygen in the Earth's atmosphere absorbs wavelengths at the edge of the ultra-violet region of the spectrum. The resulting color, which appears like a pale blue, actually is a mixture of all the scattered colors, mainly blue and green. Conversely, glancing toward the sun, the colors that were not scattered away — the longer wavelengths such as red and yellow light — are directly visible, giving the sun itself a slightly yellowish hue. Viewed from space, however, the sky is black and the sun is white.
The reddening of sunlight is intensified when the sun is near the horizon, because the volume of air through which sunlight must pass is significantly greater than when the sun is high in the sky. The Rayleigh scattering effect is thus increased, removing virtually all blue light from the direct path to the observer. The remaining unscattered light is mostly of a longer wavelength, and therefore appears to be orange.
Some of the scattering can also be from sulfate particles. For years after large Plinian eruptions, the blue cast of the sky is notably brightened by the persistent sulfate load of the stratospheric gases. Some works of the artist J. M. W. Turner may owe their vivid red colours to the eruption of Mount Tambora in his lifetime.
In locations with little light pollution, the moonlit night sky is also blue, because moonlight is reflected sunlight, with a slightly lower color temperature due to the brownish color of the moon. The moonlit sky is not perceived as blue, however, because at low light levels human vision comes mainly from rod cells that do not produce any color perception (Purkinje effect).
In laymans' terms, by Analogy
Have you ever sent waves to a log floating in a lake? If so, you probably (I hope) noticed a curious phenomenon. Waves shorter than the log's width, even if fairly high, tend to bounce off the log, and scattered back and towards the sides. Waves longer than the log's width, however, cause the log to bob up and down and go right through it, almost as though the log wasn't there.In the world of light, blue waves are the shorter (by about a half) than red waves, and instead of logs we use molecules of air (mostly N2 and O2). Air molecules are on the order of half a micron across (0.0000005 meters), as is the wavelength of white light. As noted, however, blue light is some half the length of red light. And yes, for similar reasons, the blue light will bounce and scatter off the molecule than the red, with a factor of 2*2*2*2 = 16 greater intensity ( the lambda to the fourth factor in the equations above). When the sun is high in a clear sky, this is what you see; the enhanced scattering of the blue spectrum of the was (the solar disk is yellowish-white due to the deprivation of its blue light).
We've all noticed the sun grow deeper yellow, then orange, then red as it sets or rises. This explained by the fact that, near the horizon, sunlight must pass through considerably longer and denser air. The more air it must pass through, the more blue light is scattered completely out to space. Only longer light wavelengths can still pass through (albeit attenuated), and you see from the top picture the top half of the sun is deep yellow, followed by orange beneath, then red, and then the red fades to darkness. Again, it is all a scattering phenomenon, just to different degrees. Red, having the longest wavelength, lasts the longest. However, the mistake we must not make is to assumed the light is altered somehow. If you have even watched a beam of white light pass through a triangular prism, you know that light is not changed, only separated into its constituent colors (via a different mechanism).
