Basics of Light ScatteringThere are no colorful sunsets in deep space or, so what we admire in a sunset has to do with the interaction of sunlight with the atmosphere.
For the purpose of this discussion, let us start from the picture that light is a collection of photons - light particles - that each have a given wavelength (color) which determines their energy and a direction (the straightness of a light ray). What we see as white sunlight is thus a mixture of photons at different wavelengths in the optical window, ranging from short wavelength (blue) to long wavelength (red).
(The theme generally will be not to go into the full details of Quantum Electrodynamics but to provide the important features of what happens in accessible form.)
Rayleigh and Mie ScatteringAs a light ray passes through the atmosphere, the photons it is composed of may scatter from constituents in the atmosphere. Usually that means the photon is deflected to a different direction.
There are two basic processes in the atmosphere - Rayleigh scattering and Mie scattering, which differ in their properties and crucially determine what we see.
Rayleigh scattering happens for small scattering centers - these can in principle be gas molecules, but their effect is small (the visibility in clear air is 300 km which is rarely realized in nature), more usual are dry hazes such as smoke, dust or salt crystals floating in the air.
Rayleigh scattering predominantly affects short-wavelenth photons - blue light is scattered more than red light - and it has no directional preferences, the scattered photons may end up going in any direction.
Mie scattering in contrast takes place for larger scattering centers, usually water droplets floating in the air. It affects photons of all wavelength with equal probability, but the scattered light prefers to go into a similar direction as the original light beam - Mie scattering is a so-called forward-scattering process.
As a result, Mie scattering is usually observed when one is looking (nearly) into the direction of the light - it then forms a halo around the light source. This is depicted in the following sketches:
When passing a beam of sunlight through the atmosphere, Mie scattering is thus seen around the Sun. Where is the Rayleigh-scattered light seen?
As the ray passes through the air, more and more of the blue light is scattered out of it, some green light and very few red photons. As a result, the air acquires a blue glow when one looks at it - this is the reason the sky appears like a blue light emitter and also the reason that in good view distant objects (like mountain chains) appear to vanish into blue haze (see Appendix A for an explanation why the sky doesn't look violet although that is the shortest visible wavelength and dominates the scattered light).
What remains of course when one removes all the short wavelength blue light from a ray is the long-wavelength red light. Thus, looking into the light source, Rayleigh scattering makes it appear more red.
How red? That depends on how long the ray passed through the atmosphere and on how much dry haze there is in the air. The following picture illustrates the geometry - when the Sun is close to the horizon, the path through the air is much longer than when the Sun is high in the sky.
But it doesn't automatically mean the Sun gets red near the horizon. Compare the following two pictures, both taken from the same location with the sun at the horizon.
When the air is very clean, Rayleigh scattering can be really muted and the Sun appears nearly white even at the horizon. With more haze in the atmosphere, the color grows progressively more towards the yellow/orange/red.
The illuminating rayAbove we've seen how Rayleigh scattering affects the color of the Sun disc we see. However, often most of the atmospheric moisture is really in the lower layers - often close to the ground in the convective zone. That low haze is then illuminated by the sunlight after it passed the upper atmosphere and underwent Rayleigh scattering - in other words, low haze can be illuminated by colored light. In this case, the Mie halo as well as larger regions of the sky (including clouds of course) are also affected by the shift in light color.
This, incidentially, affects all optical phenomena: Also a rainbow illuminated by a low red sun lacks the other colors, only the red part of the rainbow is visible then.
For future reference we will refer to any light ray that illuminates something we can see - a faraway mountain, a cloud or nearby haze - as the illuminating ray. This is in contrast to the ray from that illuminated object to the eye (where again scattering processes can take place) which we call the view ray or observation ray.
Continue with Optical Thickness.
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