James Dreben 
#ELI5 Why is water see through?
I'd never thought about it like this. Wow.
Rejin@Jdreben that's kinda mind blowing!
@rejinl Right?? So simple and logical but utterly fantastic.
SpaceLifeFormAnother angle to look through. (pun)
An interesting property of water is that stuff that would obscure your view, either settles out , or tends to float away. Nothing is permanently suspended.
Victor Gijsbers@Jdreben "This must be wrong," I thought, "surely other parts of the spectrum also pass through water." But no, it's exactly right.
(Well, you could nitpick and point out that not all substances would have allowed eyes to evolve that could see through it as well as we can see through water. But that's nitpicking.)
Jasper 🍉@victorgijsbers @Jdreben i think it's an deceptively easily answer. Not noticing that many substances are basically opaque to everything biological eyes could see, points that direction.
It also ignores that we evolved on land too, over 300 million years, air has a much wider range of transparant frequences, it does not explain why the frequences we see did not drift since leaving the water.
Also there is a question of why these absorption spectra are what they are in the first place.
AccordionBruce@jasper @victorgijsbers @Jdreben
No worries, there is no answer that won’t inspire a four year old to ask a follow-up
No worries, there is no answer that won’t inspire a four year old to ask a follow-up
UrbanEdm@Jdreben @Daveography Follow up question: why is water transparent at the same frequencies as air?
Biggles 20X6!@Taco_lad @Jdreben @Daveography IR, for one. Sure, it's only partial blockage, but enough that outside of the visible spectrum, you can see air.
@UrbanEdm @Jdreben @Daveography that's not what IR spotlights, lasers and cameras show.
Water vapour in air blocks IR, heated air generates IR which can obscure vision, but the primary gaseous components of our atmosphere don't block IR.
@UrbanEdm @Jdreben @Daveography carbon dioxide *does* reflect or absorb IR, but as we know, it's currently at 422 parts per million, so not exactly prevalent on a small scale, but enough to make a big deal on a global warming scale
Greg Wellman@Taco_lad @UrbanEdm @Jdreben @Daveography Big chunks are blocked. See the diagram at the bottom of this link.
https://earthobservatory.nasa.gov/features/RemoteSensing/remote_04.php
https://earthobservatory.nasa.gov/features/RemoteSensing/remote_04.php
Remote Sensing
Remote sensing is the science and art of identifying, observing, and measuring an object without coming into direct contact with it. This involves the detection and measurement of radiation of different wavelengths reflected or emitted from distant objects or materials, by which they may be identified and categorized.
@UrbanEdm @Jdreben @Daveography Even further follow up question. Why is the frequency band that both water and air are best transparent to also the band at which a very common class of star outputs most of its energy? (Hence making those wavelengths readily available to life evolving under such a star. Heck, one could continue with whether it's even possible for (a form of) photosynthesis to take place at significantly different frequencies.)
@gfwellman That seems less interesting. If air and water weren't the right amount of transparent at the right frequencies to match with our star, we wouldn't have evolved here to be able to ask the question. If it were otherwise, would life evolve in different gas and liquid mixes? Maybe.
I was more trying to get at the "what's the commonality between these two substances."
@UrbanEdm Hmm, I'm not sure there is an obvious commonality. Air is almost entirely made of non-polar molecules, while water has dipole. That makes a big difference at microwave frequencies, but not to "visible".
What I was getting at is the likelihood of life. It's very nice that such a common class of star outputs energy that can be chemically captured. If that energy couldn't reach the surface, life might be very different and likely less common. Presumably there's a minimum energy photon below which something like photosynthesis just isn't possible, so bad news for deep red dwarfs, etc.
2xfo@UrbanEdm @gfwellman
I think Greg may be alluding that only certain frequencies are really useful to help you see in water, mainly the ones that make it through our atmosphere
I think Greg may be alluding that only certain frequencies are really useful to help you see in water, mainly the ones that make it through our atmosphere
@UrbanEdm @Jdreben @Daveography air is more transparant all around: https://en.wikipedia.org/wiki/Absorption_%28electromagnetic_radiation%29#/media/File:Atmospheric_electromagnetic_opacity.svg
water: https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water#/media/File:Absorption_spectrum_of_liquid_water.png
It is transparant to some IR too. But notably most of the spectrum of ~30C objects radiate at frequencies it largely blocks.
Hugh Young-Bish@Jdreben
"specially made" suggests special creation.
I'd say "because the only eyes that were any use were the ones that could see through water (that could see the kind of light that goes through water). The creatures in the sea that could see their food and see enemies coming for them were the ones that survived, compared to their relations that couldn't."
"specially made" suggests special creation.
I'd say "because the only eyes that were any use were the ones that could see through water (that could see the kind of light that goes through water). The creatures in the sea that could see their food and see enemies coming for them were the ones that survived, compared to their relations that couldn't."
@hugh Yeah this is a good correction.
@Jdreben
Thanks. It's a heavy message that there are no purposes in nature. (Only some kind of mind can formulate a purpose.) Nothing evolved "in order to" anything, it has always been that what(ever) worked better in that time and place survived better, reproduced more, and replaced what didn't work so well, with no intention involved.
Thanks. It's a heavy message that there are no purposes in nature. (Only some kind of mind can formulate a purpose.) Nothing evolved "in order to" anything, it has always been that what(ever) worked better in that time and place survived better, reproduced more, and replaced what didn't work so well, with no intention involved.
So many dynamos
Sören@Zerofactorial @Jdreben speak for yourself 👀
Kit Bashir
Ben Aveling@Unixbigot @jpm @Jdreben Sometimes known as the Cambrian Explosion, but yes, that's probably a more accurate description.
@BenAveling @jpm @Jdreben indeed, the fascinating time of such enigmatic wonders as anomalocaris, hallucinogenia, and holyfuckitsallteethiea.
CavyherdSee also (heh) air
Okay, here's one: why do both air & water pass as many of the same wavelengths as they do?
Where do they (or do they?) differ & why?
The_LazyKnight@cavyherd @Jdreben eyes evolved to process wavelengths that can pass through both water AND air: not either/or, but both...
A good counter example is near IR and coke: coke doesn't pass visible wavelengths, but does pass IR, as does air. Stick an IR pass filter in front of a camera and you can take a photo *through* a pint of coke.
😂 TIL about Coke!
But my question was a little different: it wasn't about why the eye can see through both water & air—that was very nicely covered in your first response.
Rather, why are both air & water transparent to the same wavelengths (or at least a large overlap in the range that we can also see)?
For that matter, why is Coke NOT transparent in that range? (One presumes this has to do with transmission/emission spectra.)
realcaseyrollins ✝️@Jdreben Isn’t it just because a lot of light passes through it? And so if you throw stuff like dirt into the water, which doesn’t allow light to pass through it, the water is harder to see.
Generally, AFAIK, liquids tend to be see-through, solids tend to be opaque, and gasses tend to be somewhere in the middle.
sj_zeroIf you look at the data, water tends to absorb most wavelengths except for this little spectrum in the visible part of the spectrum.
When you get an ir or uv camera it's interesting seeing that a lot of clear things turn opaque, and a lot of opaque things turn clear.
When you get an ir or uv camera it's interesting seeing that a lot of clear things turn opaque, and a lot of opaque things turn clear.
Iwillyeah@Jdreben so is it just dumb luck that the same eyes would work for our atmosphere? Or am I missing the point?
Tido@Iwillyeah @Jdreben since the atmosphere is above the oceans, the light must first pass through it. So eyes could only evolve for a spectrum that passes both through air and water.
@tido that makes sense. If life had evolved on land perhaps the eye would have developed differently then.
Raven667@Iwillyeah @Jdreben I would think that for sun light to pass through water, it would need to transit the atmosphere first, so only frequencies that pass through both would be useful for eyes. It's a built in part of the filter for what's likely and useful
skategoat 🐐 🇵🇸@Iwillyeah @Jdreben any light that gets into water had to go through the atmosphere beforehand so it always has to work for both
Jacob Christian Munch-Andersen@Jdreben That is true, but it is not the only condition that has to be met for a wavelength to be suitable for sight. First of all for sea creatures to see light it has to be emitted by the sun, pass through the atmosphere, and pass through the water. Then it also has to be blocked and/or reflected by relevant objects in order to be useful.
@Jdreben There is also the issue of how the sensor is implemented. For instance infrared vision would be extremely useful for us above-sea creatures. But our own bodies produce infrared because they are warm, which makes it hard to filter out the relevant signal. Maybe there is a solution for infrared, but AFAIK evolution hasn't found it yet.
Knut Morå@Jdreben
I was.going to say that we've evolved to live on land, but checked, and also there water is an important absorber! not the only one but still:
https://en.m.wikipedia.org/wiki/File:Atmospheric_Transmission-en.svg
Alt: absorbtion spectrum of the atmosphere -- water is absorbing a lot at many wavelengths, but a window around optical wavelengths has little absorbtion
Matt Stewart@Jdreben
Hold on just one second.... 🤔
Hold on just one second.... 🤔
التنينوكس@Jdreben so much bullshit just to use evolution to explain basic optic science. Even the conclusion does not answer the question, just rephrased as a stament.
I am reminded of the question, "If plants don't want to be eaten, why aren't all plants poisonous?" I *think* the answer is that they are, but it's advantageous for herbivores to resist the poison, so they evolve to.
Also, "Why are there three primary colors, not two or four?" This one I'm more confident about: Most humans have three types of color sensors, so some mix of three primary colors can reproduce those sensors' response to any "natural" color.
@squeakyears @Jdreben it depends on whether you’re in an additive (starts with black; colors add up to white) color system such as a screen, or a subtractive (starts with white; colors add up to blank) one such as a printer. The latter typically has four colors; sometimes six or more to make mixing more precise. And the former these days also sometimes has four; some TVs use RGBW.
And I think you need three for a proper gamut (a triangle-like shape that shows which colors can be mixed).
Uli Kusterer (Not a kitteh)@chucker @squeakyears @Jdreben Yeah, RGB (red green blue) for screens, red yellow blue for paint/printing (though most printers use CMYK, cyan, magenta, yellow, black).
@uliwitness @squeakyears @Jdreben I was confused as a kid who was told in kindergarten that the “basic colors” are red, yellow, blue (RGB in German), then found that displays used red, green, blue (again, RGB, but a different G).
It clicked once I realized additive and subtractive are opposite starting points, so blending colors also accomplishes the opposite.
@chucker @squeakyears @Jdreben We had an arts teacher who once insisted you could mix yellow from red and green … refused to listen to my objections until we'd experimentally proven that it'd result in a bad shade of brown. No idea how that happens in that job.
MrCheeze 
@Jdreben That seems... at least incomplete, since there is much else that would be advantageous to see though but nothing can.
Stefan Ihringer
kira :3@Jdreben i have eyes that can see through bullshit #imdifferent
Sam Wade
Russ Garrett@Jdreben strikes me that it's a smaller-scale version of the anthropic principle - we only exist because this is the case so it has to be true.
Ivan Sagalaev 
@Jdreben hm... Why then vision evolved for this particular (narrow) range, while water is transparent to a lot more? I can guess why big wavelength were less advantageous, as you want to see small things with good resolution, not blurry blobs saying "your speck of food is vaguely in this cubic meter of space". But why we don't see in UV or X-ray ranges?
Zeborah@isagalaev @Jdreben Speculating: As the screencap notes, x-rays pass through flesh and only reflect off bones. If a jellyfish or octopus was transparent to you, you'd risk becoming their next meal.
@zeborah @Jdreben indeed! I missed that implication.
Also, thinking further, UV (and up) becomes irrelevant, as at smaller wavelength it should scatter fast due to all the micro-stuff in the body of the water. On the other hand, scattering doesn't mean it's invisible. There should be plenty UV in the first 10 meters…
Anyway, don't mind me, I'm just thinking aloud :-)
lothar@isagalaev
Adding to that: For evolution, "good enough" is all that is required. There was no task "conceive a maximum-range detector with ideal resolution" or the like. Just about a little better then that of the competition would do. 😉
@zeborah @Jdreben
Adding to that: For evolution, "good enough" is all that is required. There was no task "conceive a maximum-range detector with ideal resolution" or the like. Just about a little better then that of the competition would do. 😉
@zeborah @Jdreben
@lothar @zeborah @Jdreben oh, I'm well aware of that one :-) But if UV vision was giving advantage, life would likely adopt that. I'm just trying to figure out why it doesn't (apparently) give any advantage.
P.S. As for X-Ray and generally higher-than-UV energy radiation, the answer, I think, is pretty simple: it didn't exist on Earth in appreciable quantities until humans started producing those.
Alex R@isagalaev @lothar @zeborah @Jdreben it can sometimes! Bees can see in UV, which lets them see nectar better