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The Really Deep explanation of why the sky is blue is hard. Albert Einstein wrote, in 1911, that Relativity had to be used to fully understand why the sky is blue.But let's do the easier explanation.It begins with understanding something called "scattering" of light. "Scattering" means that the light hits some kind of particle, is absorbed, and then, is re-emitted. The light might get changed between being absorbed and emitted, changing in (say) brightness, or angle at which it is emitted, or even colour.It was John William Strutt, 3rd Baron Rayleigh, also known as Lord Rayleigh, who looked specifically at light being scattered by very small particles, such as molecules. Lord Rayleigh was an intellectual heavyweight, winning the Nobel Prize in 1904. But it was earlier than that, back in the 1870s, that he gave us the now-famous "Rayleigh Scattering", which explains why the sky is blue. Rayleigh Scattering works when the particle is less than about 3% of the wavelength of light. Molecules of nitrogen and oxygen are a few thousand times smaller than the wavelength of light, so the "Rayleigh Scattering Effect" is definitely happening here.The atmosphere is made from various gases - about 77% nitrogen, 21% oxygen, 1% argon, with several other gases (including carbon dioxide) making up the remaining 1%. These gases are, in turn, made up of many molecules. These molecules are very small, and very far from each other. For the incoming sunlight in our atmosphere, it's as though it travels in a vacuum, misses many molecules, and then hits a molecule and gets "scattered". This scattered light then continues travelling in a vacuum until it hits another molecule in our atmosphere. Each time the light hits a molecule, there's a time delay, during which time the light is absorbed, and then re-emitted.Now here's the essence of Rayleigh Scattering. First, the re-emitted light does not come out of the molecule in the direction from which it came - instead, it comes out mostly at right angles to its original direction. Second, the "right-angle bending effect" is much stronger for blue light than for red light, so the emitted light is predominately blue. The red light goes mostly straight ahead, and then back out of the atmosphere into deep space - and so you never see it. Third, your eyeball will register seeing a colour only if some light of that colour lands inside your eyeball.So white light will hit a gas molecule, but bluish light will come out - and at right angles to the original direction. This light will then travel in a straight line until it hits another molecule, and the whole absorb/emit/right angle/blue thing happens again. So the light that came from the sun will hit a gas molecule high in the atmosphere, and then will stutter in a zig-zag fashion down towards the ground. Each time the light hits a molecule, the light comes out mostly at right angles to its previous direction of travel - and each time, it's more blue in colour. Eventually, some light that has been through thousands of these absorb/emit situations with thousands of molecules will land in your eyeball - and then, and only then, do you get the impression of seeing blue. There you have it - the sky is blue is because of Rayleigh Scattering by the nitrogen and oxygen molecules in the atmosphere. And the ocean is blue because of Rayleigh Scattering by the water molecules that make up the water, as well as reflecting the sky
and that was the easy answer!
It is possible for the sky to go green during the very last seconds of a sunset.
Since blue colour is scattered, the last colour you see is green.
Quote Since blue colour is scattered, the last colour you see is green.So what about the red - the longest wavelength?
There will be less and less energy from the shorter wavelengths - as the light travels through a longer and longer path through the atmosphere. The green Sun is possibly a psycho-visual (!!!) effect.