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198 Black and white and the blue of

the sky

Subject

The color of the sky, namely blue, seems to be an important subject. Headline in a schoolbook:

Why is the sky blue?

Headline in another schoolbook:

Scattering of light - sky blue and evening red.

Headline in a college textbook:

Why is the Sky Blue?

The color of no other system or entity is discussed in such detail. 


Deficiencies

If one asks why the subject is treated, two answers suggest them-selves at first:

1. because one wants to explain the colors of things that surround us. Physics is relevant for that.

2. because one wants to deal with Rayleigh scattering, and the blue of the sky is an example that everybody knows.

Both answers do not seem very plausible to me.

To 1: With the same argument, one would have to look at other col-ors, or at least the most important ones: for example: Why is the wall, the snow, the cloud white? Or why is black everything that is black? Why is gold yellow or golden? But this is not done. Probably this is because it is thought that there is nothing to say: White is just when nothing is absorbed, black when everything is absorbed and yellow when blue is absorbed. By the way: If the blue of the sky is discussed, why not also the blue of the sea?

To 2: If Rayleigh scattering is considered important, so important that it belongs in a textbook, why are other scattering processes not addressed, first and foremost scattering from white objects, such as a sheet of paper or a white wall? Or is this considered trivial? But also: Why not Raman or Brillouin scattering?

One more remark on how Rayleigh scattering is explained. One talks about the role of the vibrations of the molecules, of how they act as small Hertzian antennas. But unfortunately one does so only in this context. The students learn about atomic excitations in the context of atomic physics. Are the electrons of the molecules excited here too? Into what kind of states? And if it would be so, one would not understand why the phase of the emitted coupled to that of the incident wave. Or does quantum physics no longer apply and the molecules behaves like a classical dipole antennas? The authors are honest enough to explain that the scattered light should actually interfere away, but does not do so, because density fluctuations ex-ist in the scattering air. Please do not misunderstand me. I do not claim at all that these statements are not correct. But if the waves, which actually wanted to be scattered, interfere away – would it not have been suitable to problematize this effect where it actually takes place, namely always when the light passes through a transparent body – liquid or solid – and not only when it does not take place?


Origin

Could it be that one only wants to express: Look, physics is not so boring as you always thought! If you plan to write a poem about the beauty of the blue sky, you should look into your physics book befo-re, so that you know what you are talking about.


Disposal

The interaction of light with matter can be divided into categories: absorption, refraction, reflection and scattering. In general, all pro-cesses take place simultaneously and they are wavelength depen-dent. Matter is complicated, and therefore these processes are also complicated and diverse. Among them, however, there are simple special cases. And one will discuss preferably these. These include in particular the origin of the body colors black and white. Their treatment in the classroom is suitable because the phenomena are universal.

The white of the cloudy sky, the snow, the white wall, the paper, the milk, the leaves of the margarite, a T-shirt or bed sheet always comes about in the same way: through many repeated refraction processes (in which practically nothing is absorbed), all the light that falls on a surface eventually gets out again. This is an interesting phenomenon, because one might expect that not every light ray would succeed in finding its way back to the surface. The process is called scattering - without determiners like Rayleigh, Mie, Raman, Brillouin, Compton, Rutherford or Thomson.

The process by which black comes about is similarly universal. The black of a black-painted wall, of a black-painted car, of the soot in a stovepipe, of printed letters comes about by substances which, if they are present as a smooth surface, reflect. Reflection, however, basically does not work without absorption (in contrast to refraction). You can see it clearly if you put two mirrors opposite each other and look into them at an angle. The light reflected several times be-comes weaker and weaker. If you now grind such a material, the light initially has a similar behavior as with white bodies: it is reflect-ed back and forth in a disorderreflect-ed zigzag pattern. With each reflec-tion, however, it loses intensity, so that it runs dead in the material. So we do not use any molecular or atomic physical interpretation for the colors black and white. By the way, the blackest black is ob-tained by poking a small hole in a cardboard box, such as a shoe box. The hole is black even if the inside walls of the box are white.

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