The sky appears visibly translucent until it is blue. Is that to the border of the troposphere with the stratosphere? After all, it’s black behind the blue, isn’t it? The black is not visible. I sometimes hear airplanes overhead and don’t see them? Are those behind the blue part? I read something about raleigh scattering? Is that about between 30 to 70 km that I can observe and no further? It doesn’t seem infinite to me.
Answer
The sky is not blue in one particular place. The (white) light from the sun consists of different wavelengths (colours). The blue color interacts best with the air molecules of our atmosphere and some of the blue light is ‘scattered’ (bounced off) by the air molecules in all directions. This is what is called Rayleigh scattering. If you look directly at the sun, you see the white light that is missing some of the blue, so the sun appears yellower/redder than if we were to look at it outside the atmosphere. If you look in a different direction, you only see that scattered, blue light.
So you understand that the blue light comes from ‘wherever there are air molecules’. The effect of Rayleigh scattering is small, so you can only observe it if you have a sufficiently thick layer of air.
Can you see beyond the atmosphere now? It all depends on whether the object reflects light in your direction or not. If the object is in the atmosphere, it will always reflect light, as the scattered light travels in all directions. If the object is outside the atmosphere, it can still reflect light from the sun in your direction and thus be visible. For that reason, you can observe the ISS regularly. The intensity of the sun is usually so great that it dominates the reflection, so that we can usually only observe it at night.
Answered by
Prof Walter Lauriks
Physics Acoustics
Old Market 13 3000 Leuven
https://www.kuleuven.be/
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