James Dreben 
#ELI5 Why is water see through?
I'd never thought about it like this. Wow.
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
Pete 
@isagalaev @Jdreben it’s not just that, but the wavelengths of visible light are also the only ones that are capable of causing the kind of changes in pigment molecules that we use in our eyes—it would not really be possible for life as we know it to use other wavelengths (besides a tiny bit of ultraviolet and infrared for insects) — microwave or x-ray eyes are out of evolutionary reach.
@pmcarlton @Jdreben now that feels backwards. If reacting to microwave gave us advantage, we would evolve vision based on different biology rather than existing eye chemistry. Like, say, sensitivity to temperature increase created by microwave radiation. It's much harder to show that such a thing *could not* theoretically evolve.
@isagalaev @Jdreben no, what I'm saying is that a biological discriminator of slightly different wavelengths of >700 or <400 nanometer EM radiation does not exist, but for >400,<700 it uniquely does exist in the form of rhodopsins, gfp, etc.
@isagalaev @Jdreben although one should never say something is biologically impossible since evolution is so much smarter than we are …
@pmcarlton @Jdreben yeah, I was thinking about bats just now :-) Their hearing is actually more like vision, but built on a completely different principle using ultrasonic air waves which should allow them to "see" with sub-centimeter precision. So there.
@pmcarlton @Jdreben in fact, if I'm reading Wikipedia correctly[1], and 20 KHz correspond to 1.9 cm wavelength, then bats hearing up to 200KHz can discern down to 1.9 mm objects!
