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Basically im asking if there's anything special about visible light other than the fact that we use it to see colors. If we saw in another wavelength, would it still be possible to see colors like we do now? Does visible light have something special about it that lets us see a variety of different colors?

Qmechanic
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eclair_1661
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    There is one thing that's special about the particular frequency range of visible light: it sits right at the border between non-ionizing and ionizing radiation. Above the visible light range, UV radiation quickly starts to become ionizing, commonly known as the cancer-causing type of radiation, capable of knocking electrons off atoms and causing tissue damage to our bodies. There is only a very thin gap ("near-ultraviolet") between visible light and that boundary. Animals all see light in approx the same range, though with different numbers of colors. Maybe there's an evolutionary reason? – thomasrutter Feb 09 '20 at 23:27
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    @thomasrutter isn’t it because visible light is bounded in one side by (as you say) ionising and on the other by being able to permeate water (which is where eyes likely evolved)? – Tim Feb 10 '20 at 01:00
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    The only thing "special" about visible light is that human eyes have photoreceptors that respond to red, green, and blue wavelengths. Most mammals see only green & blue, while many birds, fish, and insects can see into UV and/or IR ranges, so what's "visible light" depends on your species :-) At a deeper level, to be "visible", photons must have wavelengths that can cause chemical changes in receptor moleculesm so there's a potentially visible range between "too weak to cause a change" and "so strong it breaks down the molecule". – jamesqf Feb 10 '20 at 03:25
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    If we saw in another wavelength, would it still be possible to see colors like we do now? This part really isn't answerable. You are asking about a hypothetical situation, so really anything is possible once you say assume reality is different. – BioPhysicist Feb 10 '20 at 05:18
  • astrophysics / biology anecdote - you shouldn't wonder that there are eyes in some of the holes in the fur of a feline - you should wonder that there are holes at all – eagle275 Feb 10 '20 at 16:03
  • @Aaron Stevens: I think it is answerable. If a species has photosensors that respond to different wavelength ranges ("cones") rather than just light ("rods"), then it will be able to distinguish between the responses of those sensors, and the brain will interpret those differences as "colors". Of course we can't know what the actual experience is, but then even humans seem to react differently to colors, having different color preferences. See e.g. https://en.wikipedia.org/wiki/Qualia – jamesqf Feb 10 '20 at 18:23
  • @jamesqf Right, that is one possibility. But I could easily say with the hypothetical premise "If we could see other frequencies, maybe we couldn't see in visible anymore". It is a hypothetical scenario where either could fine. – BioPhysicist Feb 10 '20 at 18:39
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    Does this answer your question? Is there a physical reason for colors to be located in a very narrow band of the EM spectrum? – StayOnTarget Feb 10 '20 at 21:23
  • @Aaron Stevens: But it's not just "we" - meaning humans. Other species may have more than 3 types of photoreceptors, and use them to discriminate "colors" that humans can't see. Or they might have 3 types of receptors, but tuned to parts of the spectrum that aren't what humans consider "visible". Bees, for instance, don't see red, but see ultraviolet, so bee-pollinated flowers that appear plain to us often have ultraviolet markings that serve as guides: https://www.beeculture.com/bees-see-matters/ – jamesqf Feb 11 '20 at 02:57
  • @jamesqf I never said any of that wasn't possible. The fact that there are so many possibilities proves my point. That part of the post doesn't have a single, unique answer. One can discuss the various things you have been discussing, but that doesn't constitute a definitive answer to that part. That is only the point I was trying to make. – BioPhysicist Feb 11 '20 at 03:21
  • @Aaron Stevens: The question DOES have a definitive answer: no, there is nothing special about visible light, beyond the fact that humans have receptors for those frequencies, and having receptors tuned to more than one frequency range lets you see "colors". – jamesqf Feb 11 '20 at 17:47
  • @jamesqf I don't think you fully understand what I am saying, and I don't think it really matters. Thanks – BioPhysicist Feb 11 '20 at 17:52
  • @thomasrutter That should be an answer, not a comment. – tparker Feb 12 '20 at 14:38
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    One interpretation of the distinctiveness part of the question might be, if the visible range we could see shifted +350nm (so 650-1000nm) or similar, would we still be able to see the world? I haven't seen an answer address that. – NibblyPig Feb 12 '20 at 15:35

7 Answers7

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Something special about the visible range is that water has low absorption in this range. enter image description here It’s a rather sharp dip near the visible region. Since we know that life began in water, the beings that were receptive to these wavelengths had a significant advantage over the others. Thus natural selection would have favoured these life forms over the others. This maybe the reason why we are primarily receptive to the “visible” range.

Superfast Jellyfish
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    Does that also affect the frequencies that make it through our atmosphere (with its clouds and water vapour) down to ground level? – gidds Feb 10 '20 at 00:29
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    This maybe the reason why we are primarily receptive to the “visible” range. Indeed. It's more likely that we see the visible light because it is special not vice versa. – Džuris Feb 10 '20 at 01:22
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    @gidds indeed. Check https://en.wikipedia.org/wiki/Infrared_window – Superfast Jellyfish Feb 10 '20 at 05:11
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    One thing about the near UV region - plenty of insects and even some humans have a fourth type of cone that is sensitive in that region, and the main reason we can't see near UV is that the lens blocks the light from entering the eye since it apparently has too much energy and would damage the retina. And the infrared region has the opposite problem - it has so little energy that if something has enough temperature to have interesting water chemistry, it's already glowing in infrared - including the interior of your eyeball. – John Dvorak Feb 10 '20 at 07:22
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    @JohnDvorak There's a big difference between thermal IR and near IR. An object has to be pretty hot before it begins to emit significant near IR as blackbody radiation. NIR is actually pretty useful for imaging. Notably, chlorophyll is very reflective in the NIR, so the difference between red and NIR reflectivity is used in remote sensing as a gauge of plant health. – Nobody Feb 10 '20 at 12:05
  • Since I looked it up; the limit for ionizing radiation is 124 nm, so what's marked as ultraviolet in this diagram would damage eyes. – user126527 Feb 11 '20 at 08:35
  • @John_Dvorak you might like to know that my father underwent pre-implant cataract surgery, and it saved his sight but removed his green-blue-violet colour vision from human normal. He could see "bee lines" on red flowers, which reflect brightly in the near-UV that bees can see. – nigel222 Feb 12 '20 at 13:02
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The range of visible light wavelengths has a special property that makes it the commonly used range for all life forms on the Earth:

It is the range of electromagnetic wavelengths that are short enough to be conveniently handled by cell sized detectors and that can pass through the atmosphere.

The Earth's atmosphere is not transparent at all wavelengths, and living tissue is also not transparent at all wavelengths.

There are small ranges ("windows") of electromagnetic wavelengths for which the atmosphere is transparent. There is also (as far as I can tell) just one window where biological tissue can be transparent.

This diagram from the Wikipedia article on the "optical window" shows the available ranges:

enter image description here

Really, there are only three ranges that could be useful:

  1. 300nm to 1100 nm (low UV to infrared - the visible light range.)
  2. Around 10 micrometers (terahertz range.)
  3. From around 3 cm to 12m.

Those are the ranges of "light" we can receive from the sun at the Earth's surface.

Visible light is the intersection of the wavelengths we can get from the sun and the wavelengths that can be conveniently used by biological processes.

The terahertz range doesn't pass through any living material, and the centimeter (and longer) waves are too long for convenient detection by biological processes. That leaves only the range we call visible light.

Wikipedia has an article on the "optical window for biological tissue." You can only make eyes out of things that will pass the light (lenses or just the pupil) and then you have to have something that will catch it and react to the absorption (retina.)

JRE
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is there anything special about visible light other than the fact that we use it to see colors?

We can see light with wavelengths from $390$ to $650$ nm because in our eyes we have photoreceptor cells which are sensitive only for these wavelengths. If the photoreceptor cells were sensitive to other wavelengths, then we would be able to see those.

Does visible light have something special about it that lets us see a variety of different colors?

The special thing enabling us to see different colors differently is, that in our eyes are more than one kind of photoreceptor cells. Actually we have 4 different kinds, each having their absorbance in different wavelength ranges. According to Wikipedia - Photoreceptor cell - Humans and the image below there are 3 different kinds of cone-shaped and 1 kind of rod-shaped photoreceptor cells.

enter image description here
(image from Wikipedia - Photoreceptor cell - Humans)

If we had only one kind of photoreceptor cells, then we would not be able to distinguish between colors. For example, yellow light of a certain brightness would appear to us like red light of the same brightness. We would probably perceive everything as white, black, and various shades of gray.

Thomas Fritsch
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    what kind of filtering effects, if any, do the "absorbance-frequency" (color) curves represent? I mean is this some kind of reactive resonance as in an LC filter, and if not then what causes its shape? – hyportnex Feb 09 '20 at 13:53
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    @hyportnex Yes, it is resonance between light frequency and the structure of the photoreceptor molecule. When the frequency fits well, then absorption is high. – Thomas Fritsch Feb 09 '20 at 14:15
  • hm I learned that its actually 2 kinds of cells sensitive to 2 overlapping frequency-bands (red-green, blue-yellow) - which explains the high sensitivity to yellow-green as both receptors engage - and also explains a lot of the color-vision defects (red-green weakness ... one kind of receptor is genetically disabled .. likewise for blue-yellow weakness .. ) – eagle275 Feb 10 '20 at 15:30
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    @eagle275 hmm... Photoreceptor cell - Humans says there are 3 kinds cones and 1 kind of rods. – Thomas Fritsch Feb 10 '20 at 15:57
  • @eagle275 sometimes the discussion of anomalous trichromacy may completely omit S-cones from consideration. But that's just because they are not relevant to the anomaly: tritanomaly is rare and hasn't been solidly confirmed to exist, while prot- and deuteranomaly are not only much more common, but also result in significant reduction of ability to distinguish colors in the red-green area (see pictures in section 2-3 of this page). – Ruslan Feb 10 '20 at 19:43
  • All in all the colors we differ are kinda "learned" you don't know by natural instinct, that "red" is red - a butterfly or bee with ability to see UV-A see's the colors different. Or a duck - can differ more green-tones than we humans can differ all colors - although hard to compare as ducks and butterflies don't express themselves in comprehensible ways. – eagle275 Feb 11 '20 at 13:00
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    I'm tempted to downvote because this does not really answer the OP's question. Yes, obviously we have receptors for visible light, but that's a tautology. The interesting question is whether this special frequency makes it easier to have chemical receptors for it. One aspect is Thomas Rutter's comment that at shorter wavelengths radiation becomes ionizing, making it hard to deal with in live tissue. But bees see UV... And what about longer wavelengths? – Peter - Reinstate Monica Feb 11 '20 at 13:46

  • "We would probably perceive everything as white, black, and various shades of gray" Or maybe not? Maybe we would see things in shades of red, orange, green blue, purple, magenta, or a completely different color scale that maps red to high intensity and blue to low intensity. What "color" we see is just made up inside our head, it is an interpretation of reality.

     – Krupip Feb 11 '20 at 21:04
  • @Peter-ReinstateMonica In point of fact the photoreceptors themselves remain reasonably sensitive well into the infrared. But the cornea and lens on most humans filter the low frequencies. There are occasional mutants however who can see lower-than-usual frequencies. – Perkins Feb 11 '20 at 21:08
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Yes, There is something very special about visible light. Each photon of electromagnetic radiation is a packet of energy that gets delivered to a single atom when the photon is absorbed. If that atom is part of a molecule, the energy that it absorbs can trigger a chemical reaction.

Your body is a big bag of chemicals, and your life processes are all chemical reactions. Photons that have sufficiently high energy can injure your tissues. You need protection from them. Your epidermis, and your hair (if you have any) help with that: They protect you from a lot of the shorter-wavelength (ultraviolet) radiation from the sun.

But your nerves and the retinas of your eye are chemical systems too. Your retinas detect light when the photons trigger chemical reactions in proteins in them.

Visible light is the range of photon energies that are sufficiently high to trigger the most delicate of chemical reactions, but not so high that the tissues of your eye would be damaged by them. So it should come as no surprise that that is the range of energies for which we evolved the ability to see.

Solomon Slow
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    That doesn't explain why insects can see in UV and snakes can see in IR. – Lawnmower Man Feb 09 '20 at 23:21
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    @LawnmowerMan True, but insects UV vision is only fairly low energy UV, and human retinas can also detect UV, but the lens in our eye is opaque to UV. As for snakes, their eyes don't see IR, they have special Pit Organs for that task. – PM 2Ring Feb 10 '20 at 01:53
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    The answer claimed that humans see in the visible portion because of energy levels. Clearly, biology is capable of vision at both UV and IR energies, so that explanation is left wanting. The real reason is because the sun produces the bulk of its energy at visible frequencies due the blackbody curve and its temperature, and the atmosphere is mostly transparent to those frequencies. Biology will find a way, which is why there are bacteria which can utilize gamma frequencies. – Lawnmower Man Feb 10 '20 at 02:22
  • It seems to me that the question is essentially "Is it visible because we can see it, or can we see it because it's visible?" – Barmar Feb 10 '20 at 09:02
  • @barmar, It almost seems as if you are asking about the definition of the word, rather than a particular band of wavelengths. – Solomon Slow Feb 10 '20 at 13:05
  • I didn't ask the question. But it seems like he's asking whether this band is just what our eyes happen to be tuned to detect, or is there something special about this band that we evolved to detect with our eyes. – Barmar Feb 10 '20 at 13:10
  • This is definitely the most complete answer. Technically this special region includes wavelengths slightly outside of the visual range (near UV), but I don't think that detracts from the answer much. The human visual range is what it is because the three photoreceptors span the majority of the range that is possible for this type of sensing mechanism. Note that the IR sensing mechanism for snakes is completely different - the receptors in the pit organs respond to heat, not directly to photons. – JaredL Feb 11 '20 at 02:09
  • JRE's answer explains why the near-visible frequencies are less relevant, but this question is the key to understanding why the visible frequencies are most relevant. – MSalters Feb 11 '20 at 13:36
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To combine reasons given in other answers (and a comment):

Chemical reactions For us to detect light, it has to trigger chemical reactions in our photoreceptors. This less energetic the photon, the harder it is for it to react with chemicals. But if it's too energetic, it can damage our bodies. (However, there are some animals, such as electric eels, that can detect EM waves at frequencies much lower than visible light).

Interaction with water Visible light is absorbed by water less than other frequencies. This is relevant not only for aquatic animals, but also for land animals, as our eyes are filled with fluid.

Solar Spectrum You can see here that solar power peaks in the visible range. Our eyes are optimized for detecting sunlight bouncing off of objects. Other animals have sense organs for detecting other sources; for instance, the pit viper can sense infrared radiation, which is generated by living beings.

Acccumulation
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  • The Solar Spectrum argument is bad. It's based on arbitrary choice of parameters (wavelength) to plot spectral density. When plotted as a function of frequency, the peak is in IR. See here for in-depth discussion of this. – Ruslan Feb 10 '20 at 20:03
  • @Ruslan I think that you're demanding an unreasonable amount of precision from rough explanations (considering that there are clearly multiple effects at play). Even if you choose to plot as a function of frequency, then the peak is within a factor of two of the human eye's peak sensitivity. Considering that the observed EM frequencies on Earth span 20 orders of magnitude, even the max-frequency prediction is spot on. – tparker Feb 12 '20 at 14:42
  • @tparker I'm just trying to slow down spreading of this misconception about spectral densities and evolution of eyes. – Ruslan Feb 12 '20 at 14:45
  • @Ruslan I guess it's a matter of opinion, but I don't think it's actually a misconception. I think it's an important piece of the full story, albeit one with some subtleties. – tparker Feb 12 '20 at 15:24
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There is nothing special about visibile light, meaning that the electromagnetic radiation at the frequencies at which our eyes are sensibile is not intrinsically different than that at higher/lower frequency. It is just that, for some biological reasons, our eyes developed a sensibility for the electromagnetic radiation in the frequency range that we commonly define light. It is our brain that recognizes different wavelengths as different colors, but in principle, if our sensors would be sensible in another frequency range, we would probably associate colors to other frequencies.

Andrea
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    Say we will be sensitive to IR. We would have another concept of what color means, I guess. To complete the answer only. Well defined colours can be only associated with the actual visibile, as for is in this range that atoms and molecules undergo rather distinct and defined transitions. IR would be likely seen as a dirty color while high energy would be incompatible with life anyway. – Alchimista Feb 09 '20 at 13:13
  • Actually, the reasons are due to planetary astronomy and the fact that both the sun emits a large fraction of its radiation in the visible range, and our atmosphere is mostly transparent at those frequencies. – Lawnmower Man Feb 09 '20 at 23:20
  • That meaning of "sensible" is archaic. Although technically correct, most people won't understand it nowadays. "Sensitive" would be a better choice. – CJ Dennis Feb 10 '20 at 05:22
  • well .. "technically" our eyes developed primarily to a range of frequencies that our sun delivers in abundance .. luckily for us the radiation maximum was in the yellow-green area when our type of eyes were "found" by evolution ... nowadays its more in the green-blueish frequencies as the sun slowly gets brighter when it gets older ... – eagle275 Feb 10 '20 at 15:37
  • @eagle275 the location of maximum in the spectral density (which is a distribution) is incomparable with peak of eye sensitivity (which is a per-point function). Solar spectral density as a function of frequency has maximum in IR. See this page for more details. – Ruslan Feb 11 '20 at 08:36
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A very special property of the visible light is that it is in a special relation to chemistry. It has the energy specter of most chemical processes.

Lower-energy light (IR) is almost incapable of inducing a chemical process of any kind.

Higher-energy light (UV) is not selective of the chemical bonds it cracks. That also gives us the upper limit of transparency for most substances (including, but not limited to, air and water).

And in the middle, there is the visible light - capable of selectively making a chemical alteration in the substance. That's why it is also possible to engineer a molecule that is sensitive to a particular wavelength (and a lot of molecules are so without any engineering effort).

So it is not only our (and most animals') vision. It is also why plants use the same visible light for their energy needs.

It is also a lucky coincidence that our star has a maximum emission in these wavelengths.

fraxinus
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