Why does the Visual Spectrum exist?

[transreality]

The visual spectrum is so narrow in comparison with the total, does it really exist as a discrete unit, or is its existance an artifact because our eyes are tuned to those frequencies for ecological reasons?

Is it possibly rather a basal sub-unit of the UV spectrum? Or is it some sort of blending, hybrid, or interference zone between infra-red and ultraviolet? If the latter is the case, do similar zones exist between other parts of the spectrum, such as between UV and x-rays? Do stars really emit most of their total energy output in the visual range?

[Shaula]

Stars at about 6000K tend to peak in the visible. That covers a lot of the small to mid-range main sequence stars (high K to low F stars). Smaller stars peak in the IR, larger stars peak in the UV. Most stars are actually small M class ones which peak in the IR. But visible peaking ones make up about 15-20% of stars.

There are not really hard and fast boundaries between a lot of the spectral regions. The visible region does, however, also correspond to electronic excitations (without ionisation) which means that it can alter some chemical reactions - which is how we detect it with our eyes (and how plants photosynthesise).

Generally IR = molecular vibrations, UV = ionisation and excitation of atomic valence electrons. Visible does sort of blend into UV to a degree but it is at the 'safe' end of the spectrum.

[tusenfem]

Nope, it is not a "discrete unit" it just happens to be the frequency range that our eyes are sensitive to.
Were you another animal than Homo Sapiens Sapiens, you would be able to see another frequency range, e.g. further into the infra-red for example snakes and mosquitoes.

The visual spectrum is not a "basal sub-unit of the UV", all terminologies are rather well defined in frequency range. Here is a pic from wiki that shows the different regions in the electromagnetic spectrum, from radio to gamma.

For most stars the peak in their BB spectrum lies in the visual range.

[chrlzs]

The visual spectrum is so narrow in comparison with the total, does it really exist as a discrete unit

No.

or is its existance an artifact because our eyes are tuned to those frequencies for ecological reasons?

Pretty much, yes.

As noted, other creatures have different ranges of perception, and we evolved to sense those frequencies as they give extremely useful info about our environment, for our survival. There are limits, I would imagine, to what our biology *could* evolve to detect.

And then there's the question of colors, which {sorta} exist nowhere except in our minds...

[Swift]

Generally IR = molecular vibrations, UV = ionisation and excitation of atomic valence electrons. Visible does sort of blend into UV to a degree but it is at the 'safe' end of the spectrum.

As Shaula says, a lot of chemical reactions, changes in molecular configurations, and changes in oxidation state have energies in the range from IR to visible to UV, so, for chemistry-based beings, in is not surprising that our vision system is sensitive in such a range. But there are creatures on Earth who can sense in either the IR or the UV, so there is even a little less "magical" about what we call the visible spectrum.

[profloater]

It seems the evolution of plants stimulated the evolution of colour vision. First flowers which fluoresce in uv light encouraged insects to see in UV, then the evolution of fruits where the plants exploited larger animals to distribute their seeds and animals learned to distinguish ripe fruits. The plants donate valuable sugars and colour vision is essential to find the ripe fruit. Birds and humans have good colour vision but meat eaters tended to stick with mono vision which is superior in low light conditions (hunting).

[Jeff Root]

The different bands of the electromagnetic spectrum
are distingiushed or defined in various different ways,
most of which allow a great deal of overlap.

The visual band is defined by what humans can see.
It is visible because it can change the electron excitation
of some chemical substances, as in photography.

Infrared is anything longer than what humans can see,
but shorter than microwaves. Infrared waves are short
enough to be dealt with by lenses and photography.
They have enough energy to be detected as individual
photons by appropriate detectors.

Microwaves are too long for lensing or photography, but
short enough to be handled by waveguides, and long
enough to be detected by radio receivers. They don't
have enough energy to be detected as individual photons
or to cause electron excitation in chemical substances.
They are generally absorbed by nonmetals.

Radio waves are too long to be conveniently handled by
waveguides. They generally pass through nonmetals.

Ultraviolet is anything shorter than humans can see,
and longer than X-rays. Ultraviolet generally is able
to ionize many chemical substances. The shorter the
wavelength, the more substances it can ionize.

X-rays are capable of penetrating matter that absorbs
ultraviolet. The shorter the wavelength, the greater
the penetration.

Gamma rays have the same properties as X-rays, but
often originate in atomic nuclei rather than in electron
shells, and generally are shorter, more energetic, and
more penetrating than X-rays.

-- Jeff, in Minneapolis

[Jeff Root]

The peak of the Sun's output is in the middle of the
visible range, but a greater total energy is emitted in
the infrared, because that band is so much wider.

-- Jeff, in Minneapolis

[transreality]

It occurred to me that possibly the visual spectrum was the result of being where photons emitted by ionisation levels in atoms may be clustered, but it may not be that particular. Does the phenomenon of white light spreading into the constituent colour bands through a prism, does that occur in other spectral ranges. Is there a UV equivalent of white that can be split into particular discrete UV frequency ranges? Would these bands occur through the entire spectrum at the same resolution as they do in the visual range, if we were as sensitive to those parts of the spectrum?

[WayneFrancis]

The visual spectrum is so narrow in comparison with the total, does it really exist as a discrete unit, or is its existance an artifact because our eyes are tuned to those frequencies for ecological reasons?

Is it possibly rather a basal sub-unit of the UV spectrum? Or is it some sort of blending, hybrid, or interference zone between infra-red and ultraviolet? If the latter is the case, do similar zones exist between other parts of the spectrum, such as between UV and x-rays? Do stars really emit most of their total energy output in the visual range?

This has probably already been said but...
The visual spectrum is just a classification by us. Just like humans using a base 10 number system isn't something intrinsic with maths universally but how a species with 10 digits might start using the concepts of numbers. If we only had 8 fingers, including thumbs, on our hands we'd probably be counting in a base 8 numbering system.

There is nothing special about photons in the visible spectrum beyond the fact that we humans have evolved to "see" this spectrum. Some animals can see photons above or below the "visual spectrum". Essentially their visual spectrum is different from ours. It is just like sound. We've evolved to hear a range of frequencies but this doesn't mean there is anything special about this range and there is nothing blocking sound from being created at frequencies higher or lower then the audible range.

The different names we give different ranges are just human constructs to help us classify things. There is a purpose as energy levels can dictate what the photon can do. For example microwave and radio waves and visible light isn't strong enough to break apart molecular bonds. UV starts messing with molecular bonds and this is why to much UV can be dangerous.

Some stars are cooler then our sun and thus have their peaks shifted down to the red end of the spectrum. You would expect life around those types of stars not to be able to see blue colours for the most part simply because there wouldn't me many of those photons coming from their sun. Like wise some stars, like O types, are much hotter and have peaks in the UV and you might not expect as many creatures to see colours down in the red end of the visual spectrum.

[WayneFrancis]

It occurred to me that possibly the visual spectrum was the result of being where photons emitted by ionisation levels in atoms may be clustered, but it may not be that particular. Does the phenomenon of white light spreading into the constituent colour bands through a prism, does that occur in other spectral ranges. Is there a UV equivalent of white that can be split into particular discrete UV frequency ranges? Would these bands occur through the entire spectrum at the same resolution as they do in the visual range, if we were as sensitive to those parts of the spectrum?

There is no such thing as white. It is just a construct of our mind when enough photons of various frequencies come from a given direction. I can make a white light from as little as 3 colours. We do this all the time. Your monitor only displays red green and blue. But by varying the number of photons from each of those 3 frequencies we can produce any colour a typical human can see. There are some people that are full tetrachromats and can process 4 distinct colour bands. Add to this most of what we "see" is actually constructed by our mind. This is why optical illusions often work. We can trick the mind into thinking one colour is actually some other colour very easily.

Is can be hard to test for some types of colour blindness without very well thought out tests.

So basically a prism works because the photons are already individual frequencies. The prism just alters the photons path based on the photons frequency spreading the light out from a point to a band. Use a prism on a white LED light and you get a much different result then you will from using the prism on light from the sun.

[transreality]

So say for a colour such as yellow (with no red or blue component) is there really no real distinction to what we see as yellow? If our range of vision was equally broad but located up in upper ultra-violet (for example), would we either see no yellow, or else would we assign 'yellow' to some different frequency range, and what we think of the visual range would be just part of the IR (or UV) spectrum.

[KiwiBiggles]

This has probably already been said but...

Wayne has summed this up nicely I think.

There was a scene in Battlestar Galactica when one of the Cylon characters (fancy lifelike robot badguys) rages that humans created him in their image - but he wants to see a supernova in gamma rays...

Pretty cool scene.

Then they had to go screw it up and make Starbuck an angel...

[WayneFrancis]

So say for a colour such as yellow (with no red or blue component) is there really no real distinction to what we see as yellow? If our range of vision was equally broad but located up in upper ultra-violet (for example), would we either see no yellow, or else would we assign 'yellow' to some different frequency range, and what we think of the visual range would be just part of the IR (or UV) spectrum.

Take a look at http://en.wikipedia.org/wiki/Color_blindness specifically the colour chart for people with tritanopia. They 'see' yellow as what most of us would think of as pink. But being that they've always seen yellow like this they could still call it yellow. It is still the same photons activating receptor cells. Just slightly different ones. It is like asking someone what colour someone's shirt on TV is but they are watching a black and white tv.

[djellison]

So say for a colour such as yellow (with no red or blue component) is there really no real distinction to what we see as yellow? If our range of vision was equally broad but located up in upper ultra-violet (for example), would we either see no yellow, or else would we assign 'yellow' to some different frequency range, and what we think of the visual range would be just part of the IR (or UV) spectrum.

Transcribe it up the spectrum. You analogy of yellow is basically exactly what the deal is with the Near IR for human eyes. We can't see it.

Who knows what we would call it. That's a moot argument.

The visible part of the spectrum is a sweet spot. It has plenty of dynamic range thanks to Stars producing it in abundance, yet it isn't especially damaging. UV is damaging, IR can be damaging. It's the goldilocks zone of the EMS.

[Jeff Root]

The most basic fact: All electromagnetic radiation is the
same. Photons differ only in frequency. (And wavelength
and energy, which are all tied together so are really just
a single variable. Knowing one tells you the others.)

We can distinguish between different bands because
different frequencies of light interact with matter in
different ways. And the matter matters. Ultraviolet,
visible, and infrared light can all be lensed, but the
lens has to made of a material transparent to the
frequencies you want to lens.

A visible spectrum from a prism or grating has various
colors because human eyes see different frequencies
as different colors. The light itself is just a spread-out
range of frequencies. Anyone not familiar with human
vision would not know that there were different colors
in the spectrum, because there aren't. The different
colors are the response of human eyes and brains to
the different frequencies.

As djellison points out, there are several reasons why
human vision works in the part of the spectrum that it
does. The frequencies are those which cause excitations
in the electron configurations of many different molecules.
That works in two ways: It means that different materials
reflect, absorb, and transmit frequencies in this range in
a wide variety of ways. It also means that chemicals
were available for eyes to evolve which are sensitive to
frequencies in this range. Fortunately for us, three
different chemicals were used to make three different
receptors, sensitive to three (overlapping) parts of the
visible spectrum, so that we can see colors. Someone
whose eyes had receptors with a larger number of
different light-sensitive chemicals would be able to
distinguish more different colors than humans can.

Another reason human vision became able to see the
range of frequencies it does is that the peak of the Sun's
spectrum is in this range, so requires the least sensitivity
to detect. A few molecules of the light-sensitive chemical
could provide a sensation of light.

-- Jeff, in Minneapolis

[Strange]

So say for a colour such as yellow (with no red or blue component) is there really no real distinction to what we see as yellow? If our range of vision was equally broad but located up in upper ultra-violet (for example), would we either see no yellow, or else would we assign 'yellow' to some different frequency range, and what we think of the visual range would be just part of the IR (or UV) spectrum.

This opens up the very deep (1) philosophical problem of qualia: http://en.wikipedia.org/wiki/Qualia

(1) For suitable values of the word "deep".

[swampyankee]

It seems the evolution of plants stimulated the evolution of colour vision. First flowers which fluoresce in uv light encouraged insects to see in UV, then the evolution of fruits where the plants exploited larger animals to distribute their seeds and animals learned to distinguish ripe fruits. The plants donate valuable sugars and colour vision is essential to find the ripe fruit. Birds and humans have good colour vision but meat eaters tended to stick with mono vision which is superior in low light conditions (hunting).

I think there is a conflation of cause and effect there. IIRC, the evidence seems to show that mammals (largely) lost color vision to permit more of the surface area of the retina be devoted to rods, which are more sensitive to low light levels (and almost unresponsive to red). Color vision was not entirely lost, as carnivores, like dogs and cats, do have some color vision, although they have terrible ability to spatially resolve colors (they probably can't see plaid). On the other hand, many animals which are in habitats which do not include flowering plants have quite elaborate systems of color vision, e.g., there are shrimp with something like 25 distinct types of color sensing cells.

There are, I think two things going on with the visual spectrum: warm-blooded animals cannot use IR, especially to detect the heat signatures of other animals, due to self-noise (the snakes with IR sensors are cold-blooded), so this puts a upper limit on the usable wavelength, and UV frequencies become biologically dangerous, which puts a lower limit on the wavelength. I've read, in unreliable sources, that people who have had their lenses removed because of cataracts can see somewhat farther into the UV than can people who have avoided that surgery, so this may argue that the light-sensing cells can detect UV but it's blocked because light in that wavelength causes more damage to the optical system than the added information is worth.