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Atmospheric light scattering: Difference between revisions
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== Light scattering basics == | == Light scattering basics == | ||
The basic processes how light scatters in the atmosphere are [http://en.wikipedia.org/wiki/Rayleigh_scattering Rayleigh scattering] and [http://en.wikipedia.org/wiki/Mie_scattering Mie scattering]. Rayleigh scattering occurs on scattering centers which are much smaller than the wavelength of light (typically the air molecules). In this limit, the outgoing light is scattered into every direction with equal likelihood (isotrope scattering), but the probability to scatter depends on the wavelength of the light - the shorter wavelengths (blue, violet) scatter more strongly. This is the cause for the color of a clear sky - there is much more diffuse Rayleigh scattering for blue light happening in the upper atmosphere than for red light, and as a result we see all the light that gets scattered out of the direct path from sun to eye as a diffuse blue glow - the sky. The same phenomenon causes the red color of sunrises - since the sun is close to the horizon, the path the light has to travel through the dense parts of the atmosphere is long and so by the time the light reaches the eye all blue light has been scattered out and only the red light remains. | The basic processes how light scatters in the atmosphere are [http://en.wikipedia.org/wiki/Rayleigh_scattering <b>Rayleigh scattering</b>] and [http://en.wikipedia.org/wiki/Mie_scattering <b>Mie scattering</b>]. Rayleigh scattering occurs on scattering centers which are much smaller than the wavelength of light (typically the air molecules). In this limit, the outgoing light is scattered into every direction with equal likelihood (isotrope scattering), but the probability to scatter depends on the wavelength of the light - the shorter wavelengths (blue, violet) scatter more strongly. This is the cause for the color of a clear sky - there is much more diffuse Rayleigh scattering for blue light happening in the upper atmosphere than for red light, and as a result we see all the light that gets scattered out of the direct path from sun to eye as a diffuse blue glow - the sky. The same phenomenon causes the red color of sunrises - since the sun is close to the horizon, the path the light has to travel through the dense parts of the atmosphere is long and so by the time the light reaches the eye all blue light has been scattered out and only the red light remains. | ||
Mie scattering in contrast occurs for much larger particles (water droplets for instance). In this limit, the scattering is of equal strength for all wavelength (i.e. pure Mie-scattered light is white), but the scattering is strongly directional - the scattered light prefers to go close to its original direction. Mie scattering thus tends to create bright white halos around light sources. | Mie scattering in contrast occurs for much larger particles (water droplets for instance). In this limit, the scattering is of equal strength for all wavelength (i.e. pure Mie-scattered light is white), but the scattering is strongly directional - the scattered light prefers to go close to its original direction. Mie scattering thus tends to create bright white halos around light sources. | ||
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As long as the light scattering effect is weak, a medium is called optically thin. The relevant measure is the ratio of the light attenuation length divided by the size of the medium which must be smaller than one, and the defining characteristic of an optically thin medium is that you can look through. This is certainly true for the upper atmosphere where visibility ranges are easily several hundred kilometers whereas the thickest part of the atmosphere is just about 30 km vertical size. Thus, a dark blue sky is actually the blackness of space, seen through the light blue-white glow of Rayleigh scattering. | As long as the light scattering effect is weak, a medium is called optically thin. The relevant measure is the ratio of the light attenuation length divided by the size of the medium which must be smaller than one, and the defining characteristic of an optically thin medium is that you can look through. This is certainly true for the upper atmosphere where visibility ranges are easily several hundred kilometers whereas the thickest part of the atmosphere is just about 30 km vertical size. Thus, a dark blue sky is actually the blackness of space, seen through the light blue-white glow of Rayleigh scattering. | ||
As clouds demonstrate quite drastically, water droplets can easily make the atmosphere optically thick. | As clouds demonstrate quite drastically, water droplets can easily make the atmosphere optically thick. In this case, light is scattered multiple times before reaching the eye, and most information on what the basic scattering process was like is lost. Dense fog looks like a uniform grey, which means there is no color information left, and no directional information where the light originally came from. We may call this regime <b>diffuse scattering</b>. | ||
Actually, it is not quite true that diffuse scattering retains no color information. A sunrise beneath an overcast cloud cover looks blue-grey rather than red, thus there are subtle color changes of the incoming light as it filters through an optically thick layer. | |||
== Atmospheric haze == | == Atmospheric haze == | ||
What makes the problem complicated to solve in practice is that the only thing that can be calculated reliably is the density of air molecules in the atmosphere, but there are only one ingredient in the light scattering problem. | |||