Our Visual Spectrum in regard to the Sun.

[gregongie]

Is there anything special about our visual spectrum (wavelengths from 380 - 700 nanometers) and the sun's radiation?

For instance, does the visual spectrum line up with the peak on the sun's planck radiation curve--or can't the sun be treated as a blackbody?
(Note: I am most likely confusing many things in the above question)

Also, why don't we see in the ultraviolet and infrared spectrums as well?

[Achernar]

Yes, the sun is a pretty good blackbody at an effective temperature of 5780K, and the peak of its flux curve (in frequency space) is in the near infrared, so it's pretty strong in the visible band. However, a lot of it also has to do with the Earth's atmosphere. There's a "window" of emissivity in the visible band that lets light get through (now see here). Lots of IR and especially UV tends to get absorbed by the atmosphere.

[Achernar]

Aha, and here you can see a spectrum of a G-type star. It's done in wavelength space, in which the peak is around 500nm. In frequency space, the peak is around 880nm.

[Uncivil]

Originally Posted by gregongie

Is there anything special about our visual spectrum (wavelengths from 380 - 700 nanometers) and the sun's radiation?

For instance, does the visual spectrum line up with the peak on the sun's planck radiation curve--or can't the sun be treated as a blackbody?
(Note: I am most likely confusing many things in the above question)

Also, why don't we see in the ultraviolet and infrared spectrums as well?

I'd guess that our vision has evolved to give us the clearest possible view in the sun's natural light, so there probably is a pretty good match. However, it might have been an advantage not to be able to see a greater range of wavelengths. For example, would seeing into the ultraviolet spectrum give us any real evolutionary advantage? Or would it just be more information for our brain to process, possibly making prey more difficult to spot, without telling us much useful?

I belive nocturnal animals have evolved to see further into the infrared, but we aren't nocturnal by nature, which is probably why we don't have this ability.

[gregongie]

Ah, so it looks like the visual spectrum lines up rather well with our sun and with our atmospheric window.

Does anyone know what the visual spectrum is like for our nocturnal friends?

[porkchop_d_clown]

Originally Posted by Uncivil

I belive nocturnal animals have evolved to see further into the infrared, but we aren't nocturnal by nature, which is probably why we don't have this ability.

Many animals, diurnal as well as nocturnal, see at least a bit into infrared. I believe dogs can, although I may be wrong.

At the other end, many insects can see light in the UV bands.

As I understand it the theory on why *humans* see the way they do is because their eyes are adapted to a couple of different tasks: recognizing ripe fruit hanging in a tree, recognizing a predator hiding in the tall grass, and so on.

[Blake]

This question has been asked before, and with some good answers given.

http://boards.straightdope.com/sdmb/...d.php?t=204482
http://boards.straightdope.com/sdmb/...ighlight=water

The key points are that:

1) Our visual spectrum and the associated pigments evolved underwater. Moreover our light receptors still lie behind an aqueous filter. Our spectrum is tightly restricted to those wavelengths that can be transmitted through water. That rules out IR and longer wavelengths as potential visual wavelengths.

2) The higher energy wavelenths have three problems. Primarily they shake organic molecules to pieces. Organisms have evolved extraordinary processes to block these wavelengths from the interior of cells. They are also hard to focus without some pretty dense, and hence expensive, lenses. UV also has limited water transmissibility.

In other words our visual systems aren’t the result of solar output. They are the result of physical restrictions. Had life evolved on a star with radically different output it’s a safe bet that life would still be using the same narrow spectrum. Simply evolving to operate nocturnally would have almost no effect.

[porkchop_d_clown]

Originally Posted by Blake

Our visual spectrum and the associated pigments evolved underwater.

Is that part of the "aquatic ape" theory? If so, the theory is intruiging but not widely accepted AFAIK. In any case, the eyes of all vertebrates are behind "aqueous" filters and many of them see in near IR just fine. For much longer wavelengths, I agree - absorption becomes a problem and beyond that the waves are too long to properly interact with pigment molecules.

In any case, there is broad variation in the frequency response of the eyes of different animals; IIRC some argue that homo sapiens only became able to see green within the past 50k years or so.

[Blake]

Is that part of the "aquatic ape" theory? If so, the theory is intruiging but not widely accepted AFAIK

No, it is part of the univerally accepted theory that all multicellualr life evolved underwater, and that the pigments those lifeforms used to detect life in turn evolved from photosynthetoic pigments from bacteria that were also aquatic.\

I’m a bit surprised that you din’t know that life evolved in the oceans. It’s a theory tha has received quite a bit of publicity over the past 1000 years or so.

[quote]In any case, the eyes of all vertebrates are behind "aqueous" filters and many of them see in near IR just fine.[quote]

Not many by any strectch. Some, a very small proportion, of terrestrial vertebrates just might be able to see into the extremely close NIR, but it is far from being conclusively proven. If you have any evidence that supports the idea that many vertebrates can see into the NIR I’d be very interested to see it.

[Blake]

Is that part of the "aquatic ape" theory? If so, the theory is intruiging but not widely accepted AFAIK

No, it is part of the univerally accepted theory that all multicellualr life evolved underwater, and that the pigments those lifeforms used to detect life in turn evolved from photosynthetoic pigments from bacteria that were also aquatic.

I’m a bit surprised that you didn’t know that life evolved in the oceans. It’s a theory that has received quite a bit of publicity over the past 100 years or so.

In any case, the eyes of all vertebrates are behind "aqueous" filters and many of them see in near IR just fine.

Not many by any strectch. Some, a very small proportion, of terrestrial vertebrates just might be able to see into the extremely close NIR, but it is far from being conclusively proven. If you have any evidence that supports the idea that many vertebrates can see into the NIR I’d be very interested to see it.

[Achernar]

Originally Posted by Blake

In other words our visual systems aren’t the result of solar output. They are the result of physical restrictions. Had life evolved on a star with radically different output it’s a safe bet that life would still be using the same narrow spectrum.

Right, that's not exactly what the OP asked, though. While other wavelengths might not have been usable for sensory input, maybe if we'd evolved around an M-type red dwarf or something, where visible light is much less abundant, we'd be less reliant on vision and more reliant on our other senses?

[Blake]

If it’s not what the Op asked exactly, it’s pretty close, and probably more pertinent. “does the visual spectrum line up with the peak on the sun's planck radiation curve”. The point is that it’s not the sun’s output that is important, it’s the portion of the output that arrives underwater, and that is not dnagerous to life.

It also directly adresses why we don’t see in the UV and IR portions of the spectrum.

Speculation on what might have happened had we evolved around another star is of course just specualtion. But it remains a safe bet that some organisms would be utilising light for the simple reason that photosynthesis would neeed to be occurring. Once you have photosynthesis you have the beginnings of the light detection systems of the human eye. After all rhodopsin goes all the way back to the bacteria. Of thoseorgansims that were using light to sense, it’s a safe bet that they would be utilising the same portions of the spectrum for the same reasons.

[porkchop_d_clown]

Originally Posted by Blake

No, it is part of the univerally accepted theory that all multicellualr life evolved underwater, and that the pigments those lifeforms used to detect life in turn evolved from photosynthetoic pigments from bacteria that were also aquatic.

I am aware of that, thanks. I thought you were referring to genus homo in particular - my mistake. As for proof about which animals can see into near IR, I'll have to do some digging; the SciAm article I read was quite some time ago.

[Desmostylus]

Originally Posted by Achernar

It's done in wavelength space, in which the peak is around 500nm. In frequency space, the peak is around 880nm.

Mind explaining what you mean by that?

[Desmostylus]

Originally Posted by Blake

No, it is part of the univerally accepted theory that all multicellualr life evolved underwater, and that the pigments those lifeforms used to detect life in turn evolved from photosynthetoic pigments from bacteria that were also aquatic.

The aquatic evolution part of your argument is unnecessary, and doesn't really lead anywhere in and of itself. It's no better than me saying that Blake obviously has fins because he evolved from aquatic bacteria.

The sensible point is the one that porkchop_d_clown made:

In any case, the eyes of all vertebrates are behind "aqueous" filters...

The absorption spectrum of water looks like this (about half way down the page). The "visible light" spectrum coincides well with the trough in the absorption spectrum.

That works out quite well for things that live underwater, too. But it's not enough to assert that humans see in that range because we evolved from things that lived underwater. Many insects managed to evolve a radically different set of visual apparatus, for example.

[DrFidelius]

Correct me if I'm mistaken, but the reference to "aquatic evolution" seemed to refer the photosensitive pigments, not the apparatus that helps focus light onto the pigments. If I am not mistaken, the pigments were developed well before any lineage built an eye around them, and are a shared primitive trait of all multicellular eukaryotes (with minor drift exceptions, of course).

[Desmostylus]

Originally Posted by DrFidelius

Correct me if I'm mistaken, but the reference to "aquatic evolution" seemed to refer the photosensitive pigments, not the apparatus that helps focus light onto the pigments. If I am not mistaken, the pigments were developed well before any lineage built an eye around them, and are a shared primitive trait of all multicellular eukaryotes (with minor drift exceptions, of course).

I'm sorry, but that's not a sufficient reason that could explain why that particular trait persisted.

The "aqueous filter" could, and on going back through the thread, I see that it was actually Blake who first suggested it. Sorry, Blake.

[bibliophage]

This Straight Dope Staff Report by SDSTAFF Karen may be of interest: Why is visible light visible, but not other parts of the spectrum?

[porkchop_d_clown]

Originally Posted by DrFidelius

If I am not mistaken, the pigments were developed well before any lineage built an eye around them, and are a shared primitive trait of all multicellular eukaryotes (with minor drift exceptions, of course).

I'm not sure that's entirely true. The basic pigments may be quite old, but isn't the human pigment for green-reception a quite recent evolutionary trait? I seem to remember that that pigment is apparently a mutation of the older red pigment, which is why red/green color blindness is common among us - it's a throwback to a slightly older genetic trait.

Sorry for being so vague, the knowledge I'm basing this on is all 10-15 years old. I did find a web reference to a paper indicating that the genes coding for the two pigments are 98% identical - which supports your point, I suppose. The only online paper I could find for photopigment evolution was behind a subscribers-only barrier.

[Achernar]

Originally Posted by Achernar

It's done in wavelength space, in which the peak is around 500nm. In frequency space, the peak is around 880nm.

Originally Posted by Desmostylus

Mind explaining what you mean by that?

I can try. It's not meaningful to talk about the flux F(lambda) or brightness B(lambda) of an object at a given wavelength. Rather, you have to talk about the brightness per unit wavelength B_lambda(lambda). They usually say it like this: the total brightness in a wavelength range lambda... lambda + dlambda is given by B_lambda(lambda) dlambda. You can also deal with brightness per unit frequency B_nu(nu). (There's nothing wrong with treating it as a function of wavelength, B_nu(lambda), but this can be confusing, so it's avoided.) Although I don't know if it's "official", I've heard this distinction called wavelength "space" and frequency "space" often enough to use it.

B_lambda and B_nu are related not by a constant conversion factor but rather by a factor of lambda² or nu² (cf. eq. 7) so they will not, in general, peak at the same value. Specifically, for a blackbody at any temperature, the locations of the peak of the two functions will differ by a factor of 1.7597806 (Wien's law).

For UV through microwaves, we tend to use wavelength, so you'll probably hear more often that the sun's output peaks in the visible rather than IR. But I think that frequency is probably more physically meaningful. Both are valid, though.

[Chronos]

One possible source of confusion, regarding "seeing" in the infrared, is that many species of snakes could loosely be said to "see" in the infrared. But they don't use their eyes for infrared, they use heat-sensitive pits in their cheeks (hence the term "pit vipers"). I don't think that this has anywhere near the resolution of true vision, though.

[Blake]

That works out quite well for things that live underwater, too. But it's not enough to assert that humans see in that range because we evolved from things that lived underwater. Many insects managed to evolve a radically different set of visual apparatus, for example.

Different yes, but also evolved from aquatic ancestors, and also using the same approximate wavelengths. Yes some insects can see into the NUV, but so can some birds. And birds of course are vertebrates with that aqueous filter. What this proves of course is that the aqueous filter isn’t all that is playing the role here, although as I made quite clear, it is important.

There is no doubt that our aquatic past plays a vital role. Perhaps our receptors for specific wavelengths are more recent in origin. But the salient point is that we still ultimately rely on rhodopsin for our visual system. And that particular pigment goes all the way back to the monerans. An almost identical pigment still exists in the purple sulfur bacteria, and that is getting pretty primitive. We have an evolutionary hurdle to overcome if we want to exploit wavelengths that aren’t useful underwater. That legacy, combined with the ‘aqueous filter’ means that our vision is more constrained by what is available underwater than what the sun produces or what makes it through the gaseous atmosphere.