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TheNick
2008-Feb-21, 06:09 AM
What is the physical reason it's helpful for us to see things that emit light in the visible spectrum and not that helpful to see the rest of the spectrum?

01101001
2008-Feb-21, 06:21 AM
What is the physical reason it's helpful for us to see things that emit light in the visible spectrum and not that helpful to see the rest of the spectrum?

Water is transparent to those wavelengths of electromagnetism, and opaque to others, and eyeballs are largely formed from, and evolved within, water. (Touched on in topic The extent of a rainbow (http://www.bautforum.com/questions-answers/69290-extent-rainbow.html).)

Tim Thompson
2008-Feb-21, 06:42 AM
... and not that helpful to see the rest of the spectrum?
What do you mean? In astronomy, seeing the rest of the spectrum is crucial. Radio (http://www.nrao.edu/), infrared (http://www.spitzer.caltech.edu/), ultraviolet (http://fuse.pha.jhu.edu/), X-ray (http://chandra.harvard.edu/) and gamma ray (http://swift.gsfc.nasa.gov/docs/swift/swiftsc.html) astronomy are huge. Why do you think the rest of the spectrum is not helpful?

triclon
2008-Feb-21, 07:00 AM
The sun give's off it's electromagnetic radiation in what is called a blackbody curve. Essentially the hotter something is, the higher wavelengths of light it gives off. A perfect example is a flame. Red flames are cool so they give off longer wavelengths of light. Red happens to be the longest wavelength we can see. Blue flames are hot, and blue is a shorter wavelength of light The wavelengths the sun gives off happens to peak in the visible light. Since our atmosphere (and water for the ocean) are transparent to visible light, our eyes have evolved to pick up the peak wavelengths given off by the sun.

bungadunga
2008-Feb-21, 07:18 AM
What do you mean? In astronomy, seeing the rest of the spectrum is crucial. Radio (http://www.nrao.edu/), infrared (http://www.spitzer.caltech.edu/), ultraviolet (http://fuse.pha.jhu.edu/), X-ray (http://chandra.harvard.edu/) and gamma ray (http://swift.gsfc.nasa.gov/docs/swift/swiftsc.html) astronomy are huge. Why do you think the rest of the spectrum is not helpful?

He probably means evolutionarily, as in "Why would our eyes evolve to be sensitive to these specific wavelengths and not others?"

Why, for that matter, are plants green? The sun emits more greeny-yellowy light than the rest of the spectrum, so why would plants reflect it?

01101001
2008-Feb-21, 07:35 AM
Why do you think the rest of the spectrum is not helpful?

I think the questioner meant "not evolutionarily helpful", so unimpactful upon survival that the ability wasn't stumbled upon by DNA and seized.

(Some insects, birds, reptiles see near ultraviolet though, so some of us, a broad us, did find other frequencies helpful.)

astromark
2008-Feb-21, 08:21 AM
Its not that it wouldn't be cool to see more of the spectrum than we do. Its just that when we as some distant past mammal wanted to survive long enough to reproduce it was necessary to see what we should be avoiding being eaten by. We never found the need to be nocturnal so we did not develop night vision as did the Owl and Cat. Some insects see light emitted that we miss. I have seen infrared imaging of flowers. Bee's have a strange view.
On this planet in this atmosphere we can see as well as we have needed to.

grant hutchison
2008-Feb-21, 08:35 AM
As others have noted, it has to be to do with what wavelengths are readily available: emitted by the sun and not absorbed by the atmosphere.
Scattering is also a consideration: visible light is scattered a little by the atmosphere, but not excessively. Longer wavelength IR is barely scattered at all, so shadows are very dark in IR. Shorter wavelength UV is strongly scattered: the atmosphere is very hazy in UV, and you'd have trouble seeing to the horizon.
Add to that the fact that a simple eye is limited to about an octave of wavelengths before there are difficulties with focus, and the mammalian eye seem to have settled on a good compromise set of wavelengths: available at high energy, allowing us to see into shadows by scattered light and also to see large distances. All very helpful for dealing with predators and prey.
Some animals use other wavelengths: IR is useful for detecting body heat; bees (who don't care much about seeing long distances) use UV to detect patterns on flowers which we can't see; some birds use UV to detect the urine of prey animals, but also have vision in the mammalian range.

Grant Hutchison

George
2008-Feb-21, 04:45 PM
Yep, expanding further....

The spectral sensitivity of the eye is a reasonably close match to the most intense portion of the Sun's spectral irradiance, which approximates a Planck energy distribution. The range of the visible spectrum, though small in comparison to the entire e.m. spectrum, is quite large in bandwidth since violet light is about half the wavelength of the longer reds we see (Grantís octave).

This relatively broad amount of visible bandwidth causes a problem for lens-type systems, namely, chromatic aberration. This problem is due to the fact the blue light will refract more than the red, causing image distortion upon the retina. Our eye, which is a lens system, avoids this problem because our "blue" color cones are not found in the acutely sensitive central region of our eye (i.e. fovea centralis). [Surprisingly, only ~2% of our color cones are the blue receptors, yet we can still see blue easily.]

We need to qualify the peak energy level of the Sun as it applies to the eye. Surprisingly, this peak is actually not in the middle of the visible spectrum, but is found at the border of violet and blue (450.5nm as seen from space). However, converting this to photon flux, which is how the eye sees and plants work, shifts this peak point to near the middle of the visible spectrum, or about 580nm, which is yellow. [I can't tell you how depressing this actual peak result was when I found it. :) Yet, in defense, itís not really a peak, but a pimple, and certainly not representative of the Sunís color, for those heliochromologically inclined. :) ]

Certainly, it is logical that evolution would be a very good explanation of the process that has gotten our eye to where it is today. Though, as others have stated, other animals have interesting variations. Birds have four color cones (tetrachromatic vs. our trichromatic vision). Whitetail deer have only two color cones (essentially blue and yellow, but no red). Interestingly, these deer have large eyes and one color cone extends into the u.v., giving the deer certain early morning and evening advantages. At least, I think so.

Delvo
2008-Feb-21, 05:47 PM
Why, for that matter, are plants green? The sun emits more greeny-yellowy light than the rest of the spectrum, so why would plants reflect it?Plants actually do have other colors of pigments, which become visible briefly in the fall in deciduous species or are permanently visible in a few random ones (like the permanently-purple Japanese maple). So they could shift how much of each they produce, if it were helpful, but they generally haven't. The other compounds might not be as good at absorbing energy and converting it to chemical energy, or might require more resources to produce/use, or might not be as durable (with molecules falling apart more quickly)... something about chemistry rather than incoming electromagnetic energy.

Lord Jubjub
2008-Feb-22, 12:44 AM
I believe the wavelength absorbed for photosynthesis is dependent upon the magnesium atom at the core of the system.

Ara Pacis
2008-Feb-22, 01:53 AM
Like Lord Jubjub said. The usefulness of visible light may have something to do with how it reacts and causes reactions that are useful instead of destructive.

hhEb09'1
2008-Feb-22, 07:49 PM
Surprisingly, this peak is actually not in the middle of the visible spectrum, but is found at the border of violet and blue (450.5nm as seen from space). However, converting this to photon flux, which is how the eye sees and plants work, shifts this peak point to near the middle of the visible spectrum, or about 580nm, which is yellow. [I can't tell you how depressing this actual peak result was when I found it. :) Yet, in defense, itís not really a peak, but a pimple, and certainly not representative of the Sunís color, for those heliochromologically inclined. :) ]So, does that make our perception of the sun yellow. Is that what you are saying?

George
2008-Feb-22, 09:07 PM
May I nominate you for membership to the new organization known as People for the Englightenment of Solar Trichromaticity (PEST)? The purpose of this new organization is to wear the science community down until, finally, someone goes up there and does something never done before in the history of mankind -- look at the bright object that illuminates the Solar System and brings energy to all life on Earth, excluding those silly tube worms, (i.e. the Sun). Using appropriate attenuation, from space, they can look at it and tell us what color that sucker is. :) [I wish I had white-faced smileys.]

Of course, one requirement of membership into PEST is that all colorful action must be done in appropriate comedic style so as to tickle, not scratch, any who might be sensitive to our exposing touch. :)

One thing is sure...The Sun Ain't No Yeller Star!

I'll gladly bet an ice cream sundae that 95% of all those who go up there, with proper attenuation, will agree the Sun is white. White like the Moon, white like the tops of clouds, white like solar projections of non-filtered telescopes and from pinhole projections, white like color images of Solar twins (e.g. 18 Sco), white like a reasonably flat distribution of photon flux across the visibile spectrum, and white like astronauts seen in space. Look at the first astronaut to walk in space. He is white.... Ed White! :)

hhEb09'1
2008-Feb-22, 09:14 PM
Is that a no or a yes? :)
May I nominate you for membership to the new organization known as People for the Englightenment of Solar Trichromaticity (PEST). I already am: Perpetually Enrolled in Sensitivity Training

George
2008-Feb-22, 09:40 PM
There are some sites that will take that yellow pimple mole hill and make a mountain out of it by claiming this pimple makes the Sun yellow. There is just way too much puss in there that needs evacuated so some reasonable healing can take place. That is the purpose of the sister orgainzation to PEST, namely the Solar Organization for Color Korrection, or SOCK. I think once we put a SOCK in it, we will muffle all the eggregious color errors regarding our host star. ;)

Ara Pacis
2008-Feb-22, 10:28 PM
The moon is white?

George
2008-Feb-22, 10:35 PM
The moon is white? It's not really white; it's just an allusion. ;)

hhEb09'1
2008-Feb-22, 10:45 PM
There are some sites Which sites? Are there other sites of the opposing persuasion?

George
2008-Feb-22, 10:52 PM
Which sites? Are there other sites of the opposing persuasion? A vast majority of the sites, science or otherwise, will claim or infer the Sun is this or that color. Yellow or yellow-white outnumber the ones that suggest it is green, orange, or blue. Simply Google for the Sun's color and you will be amazed at what is stated for the Sun's color.

Tell me what color you would like, and I can probably find it.

How 'bout peachy pink (http://casa.colorado.edu/~ajsh/colour/Tspectrum.html). :) You probably didn't know it was a girl star! :D

Delvo
2008-Feb-22, 11:22 PM
The main reason why yellow is the most popular answer is that most people don't look in its direction unless it's low enough in the sky to get dimmer than it is when it's high up there, and when that's the case, it's filtered out to yellow by the atmosphere.

GeorgeLeRoyTirebiter
2008-Feb-22, 11:37 PM
Back to the original question, the best answer I've seen comes from Kurt Nassau's The Physics and Chemistry of Color: The Fifteen Causes of Color. He claims that the visible spectrum falls where it does because it's the frequency band where photons have just enough energy to interact with electrons by boosting them to higher orbits when absorbed (they get re-emitted when the electrons fall back to their original orbits), but not enough energy to completely knock them from their atoms.

BTW, I don't remember exactly what the fifteen causes are, but IIRC, most of them were photon-electron interactions.

George
2008-Feb-23, 12:41 AM
The main reason why yellow is the most popular answer is that most people don't look in its direction unless it's low enough in the sky to get dimmer than it is when it's high up there, and when that's the case, it's filtered out to yellow by the atmosphere. Yes, that is the best explanation for it [especially when the general public has nothing else to go by]. However, it is very surprising that even the pros can't say what color that -27 mag. star is out there. :) Take a look at SOHO's answer (http://sohowww.nascom.nasa.gov/explore/faq.html#COLOR).


Back to the original question, the best answer I've seen comes from Kurt Nassau's The Physics and Chemistry of Color: The Fifteen Causes of Color. He claims that the visible spectrum falls where it does because it's the frequency band where photons have just enough energy to interact with electrons by boosting them to higher orbits when absorbed (they get re-emitted when the electrons fall back to their original orbits), but not enough energy to completely knock them from their atoms. Is this about the emission of photons, or about our reception of photons that generate color signals? I would assume the latter, since we see near Planck distributions into the UV range for hotter stars.

Ara Pacis
2008-Feb-23, 01:35 AM
They can't see the forest for the trees.

GeorgeLeRoyTirebiter
2008-Feb-23, 03:28 AM
Is this about the emission of photons, or about our reception of photons that generate color signals? I would assume the latter, since we see near Planck distributions into the UV range for hotter stars.

Actually, it's more about the re-emission of photons: the various causes of diffuse scattering that make most objects visible are photon-electron interactions that are strongest in the visible wavelengths (although many do extend into the UV or IR ranges). It's a happy coincidence that the peak emission of the Sun also happens to fall in this band.

George
2008-Feb-23, 07:13 PM
Actually, it's more about the re-emission of photons: the various causes of diffuse scattering that make most objects visible are photon-electron interactions that are strongest in the visible wavelengths (although many do extend into the UV or IR ranges). If you are refering to the emissions from the photosphere, due mainly to the photon-electron behavior in H-, then there should be nothing special about emissions in the visible band since stars with hotter photospheres have the SED peaks in the UV, and in accordance with Planck's distribution equation. I don't see any special advantage for the visible band. Am I missing something?

FriedPhoton
2008-Feb-24, 03:44 AM
The sun has no color. Color is a concept the mind made up to distinguish between frequencies of light. What is the perceived color of the sun? My vote is white. Why? Because I can see best in white light, which means that all the frequencies are available to allow me to see with the full range of my eyes. If I could only see light in the 500nm - 600nm range, then a nicely incoherent assortment of photons within that range would be white. If I could only see in the 10nm - 20nm range, then an incoherent assortment of photons in that range would be white. White is a concept, not a color. For our eyes, the sun produces white (incoherent within our bandwidth) light.

neilzero
2008-Feb-24, 04:36 AM
Some florescent lights produce white light, but the Sun at sea level is typically slightly yellow, occasionally orange or red near sunrise or sunset. To me daylight on cloudy days appears white? I suppose each of us perceive color differently ie I see only blue, no violet nor indego at the blue edge of a rainbow, but I have possibly seen violet or indigo from ultraviolet sources. I presume my violet and indigo sensitivity is very low. Neil

blueshift
2008-Feb-24, 05:27 AM
This from Gottfried Schatz, a chemist writing "Jeff's View" (not available in the United States):

"Blue may have been the first color life saw. Before cells came up with the trick of converting sunlight into chemical energy, sunlight was a threat to them because of its harmful untraviolet rays. To avoid it, single cell organisms (the archea) developed a blue-light sensor, which controlled the cells' swimming apparatus so that the cells could swim away from blue light. This sensor has two parts. One is the colorless protein archaeo-opsin, whose chain of about 250 amino acids is firmly stitched into cell membrane, spanning it seven times. The other part is a yellowish small molecule which chemists call "all-trans-retinal", but which is basically a close cousin of Vitamin A. It is firmly attached to one of archaeo-opsins's membrane-spanning regions. The protein with its retinal attached is called archaeo-rhodopsin. When blue light hits archaeo-rhodopsin, it pushes a hydrogen ion within the protein from one place to another and changes the protein's shape. In a domino-like effect, this shape change ripples through the neighboring proteins, which eventually transmit the light signal to the cell's swimming apparatus."

"The light-driven proton movement within this blue-light sensing archaeo-rhodopsin may have inspired cells to convert the sensor into an energy capturing solar cell. By fiddling with the arrangement of amino acids in the protein chain, they shifted the absorption peak towards orange, closer to the sun's maximal energy output on earth's surface. They also made the light push protons out of the archaeo-rhodopsin and all the way across the cell membrane to the cell's outside. Because the cell membrane is an electrical insulator, the positively charged protons were trapped outside the cell, capturing light-energy as a gradient of protons across the cell membrane. Cells probably already had a membrane enzyme that could pump protons out of the cell by breaking down the cell's major energy-carrying molecule, ATP. By working in reverse, this enzyme could now make ATP by letting protons flow back into the cell. Coupling these two machines allowed cells to convert the energy of sunlight into the chemcial energy of ATP."

"Cells faced a tricky problem: they wanted orange light to power their proton pump, but did not want too much noxious blue light. To solve this problem, they modified the blue-light sensor so that it absorbed best in the orange region of the spectrum and could steer them toward orange light. However, once the protien had seen orange light and helped cells to swim towards it, it turned into a blue-light sensor, which could warn cells to dive for cover when there was too much blue light. Life had invented color vision."

George
2008-Feb-24, 09:16 PM
That is an interesting hypothesis. It does raise some questions for me. Assuming the young Earth had an atmosphere 50 or more times what it is today, how much UV would have reached the surface, since some multiple scattering of blue do occur in today's atmosphere. Why was it sensitive to blue and not UV, if UV was the dangerous wavelength?

A contrasting view would be one that suggests the blue sensitivity was an attractive ability for our little friend. Perhaps it would draw it closer to the surface and find more symbiotic life forms.

The orange sensitivity is curious since water absorbs the longer wavelengths -- which is why water appears blue -- greatly reducing the energy availble in the orange and red wavelengths. The sky color, IMO, would be relatively white, though their actually might be a very slight peak (another puny pimple) in the orange, but not useful to our friend especially if it runs from the blue light near the surface.

blueshift
2008-Feb-25, 02:44 AM
I don't know if what follows helps at all but Schatz does continue and elaborates on one of his experiments. He has a Phd in chemistry. He only wrote the book from log his lab notes:

"Modern cells draw their energy mostly from respiration or from chlorophyll-based light capture, or from both. They no longer have much use for proton-pumping archaeo-rhodopsin. Still the protein persists in today's archaea and in many marine bacteria where it backs up the more modern chlorophyll-based light capture. Good old archaeo-rhodopsin may still scoop up as much as one-fifth of the light that feeds our oceans' bacteria."

"Light-sensing archaeo-rhodopsin, however, was headed for bigger things. In its relentless quest for vision, life tested the protein successfully as light sensor in some higher algae and molds. These cells respond to light, whereas mutants lacking the protein do not-they are blind. As modern cells became more sophisticated, they slightly changed the retinal as well as the protein ( or one of its molecular ancestors) so that the resulting product (rhodopsin) could interact with the sophisticated signal transmitting system through which modern cells monitor their environment. These changes altered the archaeo-rhodopsin almost beyond recognition; only the tell-tale seven transmembrane spans of the modern protein (opsin) still reveal its archaeal roots."

"...Already more than 800 million years ago, this modern rhodopsin system allowed animals to distinguish blue and yellow. Later on, insects and higher animals duplicated the gene for the yellow sensor and then changed one of the two copies, so that the changed copy responded to red or green. Further modifications of rhodopsin led to systems that could see still more colors: bees, many fish, reptiles and birds can distinguish four colors, and many butterflies as many as five- from deep red all the way into the untraviolet. Some animals can even peer into the infrared."

"In order to see in dim light, most animals also acquired yet another type of rhodopsin that is extremely light sensitive, but cannot distinguish between different colors. About 400 million years ago, many animals had three to five different color sensors as well as a rhodopsin for dim light. Early mammals that hunted mostly at night did not need to see many colors and regressed to two-color vision. It was only 35 milliion years ago that ancestors of primates and humans re-invented mammalian three color vision, perhaps because it helped them distinguish ripe from unripe fruits against a background of confusing foliage..."

Keep in mind that he does not elaborate on all of his data that backed this up because the book is a collection of essays Schatz was sending to the Federation of European Biochemical Societies. The essays were aimed at incoming freshmen at universities. My niece has a Phd in biochemistry and says that his findings back up her studies as well..That does not mean they are right and they do admit it. It just means their access to some data may differ from yours although you do raise some views worth considering.

I think archaeo-rhodopsin was sensitive to UV but perhaps blue was close enough in frequency for it to react to? All bathwater has to do is get a little warmer and kids start to scream ahead of time.

Scahtz further admits this:

"Perhaps I am telling the story backwards. Perhaps the proton-pumping archaeo-rhodopsin came first, and the blue light-sensing variety came later. Comparing the structures of the two rhodopsins does not reveal a clear-cut genealogy.."