In today’s column (http://www.straightdope.com/mailbag/mvisiblelight.html) Karen of the SDSTAFF says basically that it’s because the Sun puts out most of it’s energy in those spectra that the receptors of most creatures respond to ‘visible’ light.
While this is true as far as it goes, it neglects another vital reason for ‘visible’ light to be the spectrum of chose for vision: The Earth’s atmosphere. Any astronomer will be familiar with the concept of the earth’s atmosphere interfering with the passage of some EM spectra. In fact there are only two ‘windows’ in the atmosphere that allow for the majority of the energy to be transmitted through the atmosphere: The visible light band, and the radio band.
The prevalence of visible light as the spectra of choice is likely due to the fact that there’s so much more of it available from the Sun that radio energy. I think it’s incomplete to say that the ONLY reason for the focus on visible light is because of the Sun’s emissions.
Another factor is that a lens can focus precisely at only one wavelength. (If you have a really good 35 mm camera, note how the lenses include a separate gauge for focusing with infra-red film.) So just tuning the eyes to see everything won’t work; it has to be within a range the lens allows.
The size of the eye is another factor. Really small eyes prefer to lose red and pick up some ultraviolet.
There’s another missing factor in what wavelengths we can see… (it’s OK it I pile on poor Karen, right? It’s only what she deserves for belittling puns!)
That 's the constraints of what kind of sensors evolution can build out of organic molecules. No doubt with the proper selection pressure, creatures could evolve pigments that are sensitive to light slightly farther up or down the spectrum, but I think that it may be very difficult or impossible to build a reasonable organic pigment that detects X-rays or radio waves.
Focussing them is a related issue, too, as JWK points out. It’s fairly easy for evolution to change an eye’s size within small limits, but it can’t make a 12 meter eye on a 10 cm animal.
In fact, I think the peak output range of the sun actually has little to do with it, as the key issue is the peak range down here below the planet’s atmosphere, and whether organic sensors can work in that range.
As has been noted before, the internet is not big enough for complete answers.
The “down here” peak pretty much is determined by the peak of the sun–so I don’t see how you can say it has little to do with it. Besides, how do teeth pick up radio programs then, if you need a 12-meter pickup?
For visible, read “perceived”. Us humans can “see”, i.e., perceive the EM radiation in the 400 to 700 nm range with our eyes.
Makes sense since the EM radiation at the Earth’s surface (below the atmosphere) peaks at that range, as said before. (I’m talking about “shortwave”, not radio which is not much use for high-resolution imaging.)
Nevertheless, some critters around us can use the radiation outside that range. I seem to remember some raptors can see up into the near UV and rattlesnakes have an organ (“eye”?) that can detect IR. Correct me if I’m wrong.
The sun’s estimated lifespan is, indeed, about 10 billion years – but nearly five of those 10 billion years have already gone by. In another 5 billion years or so, the sun’ll slowly bloat up into a Red Giant (over the course of a mere handful of years, only 10-50 thousand or so), and then, after a few million years of burning hydrogen in a core-surrounding shell and helium in its core, the sun will blow its outer layers off into space and all that’ll be left is a teensy, tinsy, hot, super-dense White Dwarf.
There are three optical atmospheric transmission windows:
0.3 to 2.5 um UV to NIR most military lasers are either 1.064 um or 1.54/1.57
3.0 to 5.0 um MWIR used in most newer infrared imaging systems
7.5 to 14 um LWIR used in some older infrared imaging systems and newer target detection systems as well as commercial imaging systems and motion detection.
Peak wavelength response of the dark-adapted eye is between 0.5 and 0.55 um. Luminous efficacy has a peak value at 0.555 um. Maximum luminous efficacy occurs for a blackbody at 6600 K, which is close to the color temperature of the sun. The eye is pretty close to be perfectly adapted to the most efficient energy transmission from the sun. Likewise for the peak absorption efficiency of chlorophil.
I believe I read that the part of the EM spectrum we perceive as visible light can travel through water fairly well, while other parts of the EM sprectrum don’t. This becomes pretty important for the evolution of eyes which started underwater. By the time that sunlight got through the atmosphere and water, ‘visible light’ is mostly what’s left.
I think if the sun were to start emitting its intensest radiation in the X-ray band starting tomorrow, we’d have a lot bigger things to worry about than that we can’t see very well. sizzle sizzle
faugeres34 said:
Yes, a certain class of snakes (including rattlesnakes, and water moccassins) are called “pit vipers”. Below the normal eyes on their faces, they have depressions (pits) that are sensitive to IR. It would not be improper to think of these pits as an early or precursor eye.
One problem with IR is that the eye itself, being at body temperature, would emit thermal IR radiation. An IR eye would have to be mounted on a stalk and cooled by air convection and/or evaporation.
Detecting X-rays with organic material isn’t difficult if the flux were high enough. NaI and plastic scintillators are often used to convert X-rays to visible light, which is then detected by an optical sensor. Most commercial radiation monitors work that way. Focusing is difficult, but you could instead use a collimator to limit the view angle. A compound eye made up of many such elements would give you an image. The X-ray telescope on the SMM (Solar Maximum Mission) satellite used the exact same principle.
Of course, compound eyes. Being able to detect transmissions (heat, radio) is not quite the same as “seeing”, or you might say our skin was a precursor eye.
Electric eels can detect changes in their electric field, so it seems that waves of the correct frequency range would be detected.
It seems to me that the relevant question is not what frequency is strongest in daylight, but at night. After all, our eyes actullay reduce the amount of light getting into them during the day. So what’s the frequency of light spread? I suppose that one major source of light is the moon, and presumably since it simply reflects sunlinght, the frequency curcve is similar.
I should do what the late Carl Sagan did, and intentionally pronounce “billion” with an explosive “b” at the front so people won’t confuse it with a paltry “million.”
'Course, then, I’d end up with Johnny Carson making fun of the way I talk.
Only if it’s lunch at McDonalds. They’ve had billions and billions served, you know.