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People frequently point out that water has a pretty narrow range in which it isn't very absorbing of light, reaching a minimum at a wavelength of about 500nm:

h2o

And that our eyes have made good use of this tiny available range, through evolution:

eye

That part makes sense to me, because the eye changed to match the physical parameters available.

But if you look at the solar radiance spectrum:

solar

You'll see that its peak matches VERY closely with the range water lets light transmit through. But the sun's radiance is just roughly a black body, and the absorption of water is just something inherent to the chemical structure of water. So is it coincidence that they match up?

Thanks!

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  • $\begingroup$ It's 100% just coincidence - there's no plausible way in which these two facts could possibly be causally related. $\endgroup$ – Nathaniel Apr 8 '14 at 5:06
  • $\begingroup$ There's a similar transmission peak in the Earth's atmosphere: gsp.humboldt.edu/olm_2015/Courses/GSP_216_Online/images/…. Since only ~2% of the Earth's atmosphere is water vapor, I believe this is an independent coincidence, although I'm not sure about that. $\endgroup$ – tparker Sep 21 '16 at 7:34
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It's 100% just coincidence. There's no plausible way in which these two facts could possibly be causally related.

The temperature of the Sun isn't in any way influenced by the transmission spectrum of water, since water molecules do not exist inside the Sun (it's too hot for them to be stable); and, vice versa, the transmission spectrum of water could hardly be affected by the temperature of but one star in the cosmos, even if it does happen to be the nearest one.

One answer mentions the possibility of anthropic bias. The claim here is that (1) a match between these two frequency ranges is needed for eyes to evolve; and (2) eyes are needed for intelligent life to evolve; hence we could only have evolved on a planet where these two frequency ranges match closely.

However, neither (1) nor (2) is convincing. Red and blue stars give out plenty of light in the visible spectrum (else you wouldn't be able to see them in the night sky), so there's no reason to think eyes couldn't exist on a planet orbiting such a star, and that's without even considering that they might be made of some other substance that is transparent at the star's peak frequency range. Even if eyes were impossible on such a planet, it's hardly difficult to imagine an intelligent species that relies on other senses instead.

The only other way these two facts could be related is if the temperature of the Sun and the transmission spectrum of water share a common cause. But there are in fact other stars at different temperatures, so we can immediately rule that out.

Having eliminated all the ways in which these two facts could be causally related, we have to conclude that they aren't related at all; it's just a coincidence.

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  • $\begingroup$ Nice answer, just one question: do you know why human eye is adapted to this frequency peak if we are not living under water? Since we are living on a soil, shouldn't eye be more adapted to have largest sensitivity where air absorption of sunlight is lowest instead of water? $\endgroup$ – matori82 Oct 9 '17 at 10:05
  • $\begingroup$ @matori82 that's a fair point. I guess to a large extent that's also a coincidence as well - that range of frequencies just happens to be quite useful to see in, and quite physiologically feasible to detect. But our ancestors evolved eyes when they were still fish living in the water, and our eyes evolved from their eyes, so there's arguably an evolutionary influence from that time as well. $\endgroup$ – Nathaniel Oct 9 '17 at 13:21
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I believe this is just an coincidence, but might also have been an important factor in the evolution of life on Earth, since improves the effectiveness of photosynthesis.

But if you look at the image below (same source as the previous link) you can see that there is also a big part of the "dip" in the absorption spectra of water outside of your visible/solar spectrum (shorter wave length). enter image description here

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I think it's a kind of survivorship bias. The majority of stars have spectra that don't coincide in this way, and perhaps nobody intelligent evolved on planets orbiting those stars to ask questions similar to yours :)

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  • $\begingroup$ Hmmm, this is probably actually it. It really couldn't be any other way. $\endgroup$ – YungHummmma Jan 7 '14 at 19:29
  • $\begingroup$ (1) What evidence do you have that nobody intelligent evolved on such stars? (2) It's not as if red or blue stars don't give off any light in the visible spectrum. You can see them in the night sky after all, so eyes would hardly be impossible on worlds orbiting such stars. Conclusion: it's not anthropic bias, it's just coincidence. $\endgroup$ – Nathaniel Apr 8 '14 at 5:03
  • $\begingroup$ There's no a priori reason to believe that intelligent life needs light -- you could imagine creatures evolving using sonar or some sort of chemical sniffing to see and manipulate the world around them. $\endgroup$ – Jerry Schirmer Apr 8 '14 at 5:45
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The spectrum of the Sun's solar radiation is close to that of a black body with a temperature of about 5,800 K.

There are spectral classifications of stars, and sun type stars ( in temperature) seem to be 7.6% of the sequence. That is an appreciable probability to allow for a random coincidence with the water absorption curve even without considering the type of planets around them.

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There are some good articles on this, which of course I can't track down right now. In all likelihood, the "water gap" is only part of the story behind the evolution of our eyes' spectral sensitivity.

The retina is essentially a quantum device, meaning it detects photons when the photon energy matches excitation states in the chemicals/compounds in the retina. (as opposed to thermal absorption where the photon energy dissipates into the material structure, roughly). Now, it happens that there are lots and lots of photons in the near-infrared range of solar irradiance, but these photons are relatively low energy, and there just aren't any organic compounds with a small enough bandgap to be excited by infrared photons.

This is not to say that the utility of sensing particular wavelengths (e.g. predator detection) isn't important to the overall evolutionary history as well.

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  • $\begingroup$ Sorry, I don't think this really answers my question; maybe I didn't ask it well. I'm not so much interested in the human eye/life aspect, I'm interested in just the sun radiance vs. water absorption aspect, and if it's just a coincidence. $\endgroup$ – YungHummmma Jan 7 '14 at 19:27
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A red dwarf would have a very different emissions spectrum. Compare the action spectrum for chlorophyll as such (no other chromophore helpers) to a red dwarf's optical emission. Photosynthesis likes a red sun.

http://www2.estrellamountain.edu/faculty/farabee/BIOBK/pigment.gif http://www.sdss.org/news/releases/wddm_binary.jpg

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Perhaps I can piece all these thoughts together. Black bodies peak at a variety of wavelengths.

black body intensity vs wavelength

And our sun's evolution is expected to wander in temperature as simulated here:

sun's temperature

And during its aging there has been times when water and the radiation have not lined up. Perhaps we are here now partly because the characteristics of our environment now (wet planet, peak sun power) are favorable to make use of all this water and light energy.

Evolution tries to find the peaks for any system. Whether it is availability of food or energy (light, sound, etc.). While it seems surprising that water and our sun's black body temperature line up, that is probably not a coincidence, but rather something that evolution took advantage of. This made the fluid water ideal for transmission of the peak amount of light in our atmosphere. So our eyes are mostly filled with water and optimized for green light.

There is nothing to say that on a dryer planet or different atmosphere or a system with a different temperature star that some other solution wouldn't be more optimal yet look like a coincidence.

If we evolved without the ability of sight, perhaps on a dark planet, we wouldn't even notice that water and a 5777K star radiance spectrum line up at the peak. There are probably many other things that might peak around other temperature stars too that no one has bothered to notice because it isn't relevant to us or our environment.

EDIT: Just to clarify...I'm saying it's a coincidence that 5777K sun and water are closely aligned. And I'm saying the fact that our eyes take advantage of that fact is not a coincidence at all.

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