I recall hearing somewhere that water is very slightly blue. Is there some depth of water that would be entirely opaque to any level of brightness, and if so how deep would that be? And while that would be black from the unlit end, would someone standing at the end where the light is perceive the water as blue? And if they wouldn’t, in what sense is water blue?
spankthecrumpet, there is no reason whatsoever for you to be using weird characters in the thread title to draw attention to it in GQ. You are free to start another thread with a normal title, but no more fancy thread titles.
Colibri
General Questions Moderator
Ok, after receiving a message from the OP I have edited out the extraneous characters and reopened the thread.
The aphotic zone, the depth below which only 1% of light penetrates, begins between 200 and 1000 meters. That’s for sea water. Pure water is probably clearer.
Water is blue because of the way light is absorbed and scattered by it. The color can be a bit different depending on dissolved gases and particles of stuff in the water and that sort of thing. Sometimes it looks more green than blue, for example.
The parts of lakes or oceans that sunlight penetrates to is called the “euphotic zone” (or “photic zone”). It varies a bit depending on the water and what is in the water, but generally it’s about 200 meters. Once you get below the euphotic zone there really isn’t any significant sunlight to speak of, but technically some light does penetrate down as about 1,000 meters. The human eye isn’t sensitive enough to detect such low light levels though, so to you it would look pretty dark down there.
If you are below the euphotic zone and look up, you won’t see blue. It will look completely dark to you. The transition isn’t abrupt. As you go down, the sunlight gets dimmer and dimmer. At some point you reach what is called the disphotic zone, where it is too dark for photosynthesis (which many aquatic plants depend on) but there is still some light present. Eventually though it gets completely dark, and at that point it’s called the aphotic zone.
Water is a blue chemical. That’s the simple fact, which seems to get lost: A sufficient volume of water is colored blue.
The reason why it’s colored blue is also well known, and is related to how light behaves when passing through it, but that’s just an explanation of the fact water is blue.
That’s a rather generic explanation for any color. Water is interesting though. It’s blue color comes from vibrational stretching modes of the O-H bond. Of course vibrational modes usually absorb in the infrared region, and that is where the O-H bonds absorb the strongest. But they also absorb at 2nd and 3rd harmonics. In this case, it is the higher harmonics of the O-H stretches that are absorbing red light.
Didn’t know that, conventional wisdom is that “water is colorless.” Chalk up another lie my teachers told me. Roughly what depth of water would be required before this color becomes perceptible to the average person? And isn’t most water present in large quantities something other than pure H[sub]2[/sub]O? For instance, seawater must have tons of stuff in it to account for color, other than just the pure water. Even swimming pool water has chemicals.
Apparently, you only need a bucket (a few gallons?) to see the blue color of water, which also appears to be dependent on the overall depth/volume of water and the way light passes through it; for example, it doesn’t look darker blue at the bottom of the bucket (also note the sides of the pool). As for impurities, I know that chlorine gas is yellow/green but probably not so in water as a compound, as in swimming pools (sodium chloride, the main salt in seawater, is colorless; crystals look white for the same reason snow looks white, although compacted glacial ice is blue).
I don’t like the bucket picture. Why does it have to be floating in a swimming pool that looks as though it is painted blue (as pools so often are)? That just puts the whole impression into doubt. There is also the issue of whether the blue sky is being preferentially reflected. It isn’t clear whether the pool is indoors or outdoors but if it is outdoors the lighting appears not to be diffused by clouds, suggesting there is a blue sky, and that might be overrepresented in the light reaching down into the bucket. Certainly something is extra bluish, in the glare in the top of the photo slightly right of center.
There are laws describing how light of a given wavelength decays in intensity as it travels through a medium. The Beer law, the Lambert law, and the Bouget law all say the same thing (or at least I don’t quite see how any of them can stand apart from both others though they are typically worded differently). The thinnest layer of water absorbs all wavelengths, just not very much; and even at the bottom of the ocean, some light of every wavelength still gets through, again just not very much.
Good point Napier. The caption on the page that links to it reads:
so it’s also claiming the pool itself is so blue because of the color of water, but I don’t believe that at all.
At the bottom of that page, the external link Why is Water Blue?, the picture to the right of the annoying blinking sentence shows a picture taken through 3 m (ten feet) long tubes of water and heavy water, and the regular water has a bluish tinge, but nowhere near the blue of the pool.
It looks like white ceramic tile to me. Do you suppose they painted exactly up to the waterline, including half a white tile? That’s quite an elaborate ruse.
Also, the bucket is in the pool to demonstrate how different volumes of water appear more or less blue. They’re both blue, but the smaller volume bucket appears less blue.
And as ZenBeam pointed out, it’s an interior pool.
The water is a uniform blue right to the edge, not gradually becoming white at the wall as the water depth decreases. Also, those tiles on the side have a bluish tinge, where they’re reflecting the water. The whole picture looks consistent with white glossy tiles, and light blue flat paint everywhere below the tile.
If the pool is painted white, why isn’t the side of the pool only as bluish as the water in the bucket? Why is it just as dark as the water above the pool bottom?
We can neatly sidestep the whole “what color is the pool” issue:
When I was a teen, I lifeguarded at an indoor pool. The bottom (and sides) of the pool were brown. We had a metal shed overhead with no windows, so sunlight could not get in. Our lights were big industrial halogens with an obvious yellow cast.
Our water was still very blue.
Yes, yes, but everyone remembers that “Brown plus clear equals blue, as long as it prolongs the endless discussion”! Sheesh, where did you go to kindergarten?
Holy shit! I don’t want to alarm anybody but I just checked and the blue pool painters got to EVERY SINGLE POOL on Google image search!
https://www.google.com/search?q=indoor+swimming+pool&tbm=isch
Alarmed! You don’t know the half of it. I just now filled a bucket with ultrapure water from my labs unit and sometime when I walked away, they must have painted the inside of that bucket slightly blue. Worse yet, this building is locked with keycards and biometric fingerprints and I’m the only one in the building. That means I must have done it! I’m the Manchurian pool painter!
nm
What’s the matter, feeling blue?
Didn’t want to get into it by pointing out that your post had fuck-all to do with whether the picture on the page was a valid example or not. None of you could explain why the pool is blue right to the edge.
But feel free to continue your Jr. High comments.