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I know that even pure water absorbs some visible light enough to be noticeable over short distances. According to the file File:Absorption spectrum of liquid water.png, the the wavelength of visible light with the lowest absorptivity is 500 nm at about 0.02 $m^{-1}$ and the wavelength of visible light with the highest absorptivity is probably the red end of the spectrum at 700 nm at about 1 $m^{-1}$.

As described later, I suspect that liquid nitrogen is much less absorbent of visible light. I'm imagining up a much colder planet than Earth where we have liquid nitrogen based life. If I was one of those life forms, I would have thought of liquid nitrogen the same way as I think of water, yet when that liquid is pure, it would probably be extremely transparent to visible light. It feels so wierd how transparent something I think of the same way as water would be.

My question is

What is the absorption spectrum of liquid nitrogen?

I once saw a YouTube video with liquid nitrogen and I believe that water has a high enough absorptivity of red light that even with a low depth of approximately a decimeter, I could tell that it was clearer than water. I suspect liquid nitrogen is extremely transparent in visible light. The fine structure constant is small at about $\frac{1}{137}$ and a photon released by an electron jumping from the second energy level to the first energy level in a hydrogen atom is in the visible region. That's probably part of the reason for the low absorptivity of liquid nitrogen. Also a nitrogen molecule hybridizes in such a way that the 1s and 2s orbitals don't hybridize and each p orbital of one atom hybridizes with a p orbital of the other atom to create a bonding orbital and an antibonding orbital and all the bonding orbitals are filled but none of antibonding orbitals are. That also appears to give no way for a photon of visible light to be absorbed through the jumping of an electron to a higher energy level.

From reading the Vibrational spectrum section of the Wikipedia article Electromagnetic absorption by water, I think some of the absorption of water comes from vibrational transitions. A water molecule has 3 degrees of freedom of vibration and according to what somebody once told me, the hydrogen atoms have a really low mass. The absorption by vibrational transition probably partly extends to visible red light for that reason. A nitrogen molecule on the other hand is diatomic so it has only 1 degree of freedom of vibration. Also since a nitrogen atom is about 14 times as massive as a hydrogen atom, the absorption by vibrational transition is probably in electromagnetic radiation of lower wavelength.

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  • $\begingroup$ Lots of info on the far infrared, harder to find for visible. Not clear why you have this tagged 'particle-physics' though (unless you want to talk about Cherenkov counters). $\endgroup$
    – Jon Custer
    Commented Nov 22, 2019 at 18:42
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    $\begingroup$ @JonCuster I think that other users have expert knowledge that I don't have making it hard for me to figure out how to tag questions properly. I added a tag because I have to but I prefer an answer that totally ignores the tag. For questions asked by somebody like me, probably just the body of the text and not the tag gives a lot of information about what the question asker is looking for. $\endgroup$
    – Timothy
    Commented Nov 22, 2019 at 18:53
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    $\begingroup$ I'm wondering whether if this question got moved to chemistry Stack Exchance, there would be a higher chance that one of its users would be able to answer this question. $\endgroup$
    – Timothy
    Commented Nov 27, 2019 at 23:00
  • $\begingroup$ I didn't think properly. I should have asked this on Chemistry Stack Exchange. It's in a course called "Physical Inorganic Chemistry" that I learned how the molecules from hydrogen to neon really hybridize. I know Physics Stack Exchange can answer some questions about quantum mechanics and hybridization is explained by quantum mechanics. Would it be possible for somebody to move it to Chemistry Stack Exchange. $\endgroup$
    – Timothy
    Commented Dec 14, 2019 at 2:23

1 Answer 1

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https://commons.wikimedia.org/wiki/File:SwimmingPoolAndBucket.jpg shows a bucket full of water. I believe it is a tall bucket. I know swimming pools probably sometimes have blue tiles at the bottom to make them look bluer. The YouTube video Don't Put Beer In Liquid Nitrogen - Top Awesome Experiment on the other hand shows liquid nitrogen in a container that appears half its height and it appears to be completely transparent to visible light.

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    $\begingroup$ You can't judge such unsaturated colors from the photo: the color there depends on white balance setting used when shooting and post-processing. Moreover, you can't even compare two unrelated images, since the shooting modes are most likely different there. $\endgroup$
    – Ruslan
    Commented Dec 13, 2019 at 9:04
  • $\begingroup$ I know people do sometimes have false senses of things like when they reconstruct a false memory. Maybe they also sometimes miss see things. I really don't think I would have reported my observation of the liquid nitrogen being very transparent if it had the same absorption spectrum as water. I think the container was tall enough that if it were water, I could have seen a bit of blue colour. I made my judgement based on what it looked like before the beer went in. There were no objects in there to reflect coloured light. As the small thin layer of mist moves, my brains gains more information $\endgroup$
    – Timothy
    Commented Dec 13, 2019 at 17:55
  • $\begingroup$ about how it would have looked without the mist. I think it can be explained by the amazing power of the brain to adapt to the moving mist. I really think the brain can do things like that for some people. I've sometimes noticed that the sky appeared to be a featureless blue and then when I slowly moved by eyes by doing something like walking while looking at a power line, I could see extremely faint clouds. That's probably because my brains knows how negative after images work and when my eyes are continuously moving, my brain can deduce fine detailed information about how the sky should $\endgroup$
    – Timothy
    Commented Dec 13, 2019 at 18:00
  • $\begingroup$ really look even in the presence of after images. $\endgroup$
    – Timothy
    Commented Dec 13, 2019 at 18:00
  • $\begingroup$ @Ruslan I thought the bucket was a short bucket but I guess I was wrong. Maybe you're right and I cannot tell by observation that liquid nitrogen is more transparent. I think they also sometimes tile the floor of a swimming pool blue making water look bluer than it is. $\endgroup$
    – Timothy
    Commented Dec 13, 2019 at 18:08

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