Across the electromagnetic spectrum, 400-700 nm is a narrow spectrum of frequencies and focused in the region of short wavelengths. For example, radio waves cover a large range of frequencies unexploited by the visual system. So what biological reason is there that evolved us to use such a small frequency bandwidth for vision?

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    $\begingroup$ We can only answer "how" or "how come"? "Why" actually is quite speculative and philosophical. Why human are the way they are? Why didn't humans evolve to swim naturally or run fast (very favourable attributes)? We can just guess the "why". :) $\endgroup$ – WYSIWYG Jun 22 '15 at 10:49

Short answer
The visible spectrum has the highest energy in sunlight at the earth's surface, explaining the gross location of the visible spectrum in life on earth. The specific frequency range varies across species and can be explained by species-specific survival strategies.

When you look at the solar light spectrum at the earth's surface the visible spectrum has the highest intensity (fig. 1).

solar irradiation
Solar irradiation. Source: University of California.

So it makes sense to use the range of frequencies that is represented most in sunlight as a starting point.

Then the question becomes, why do humans utilize approximately 400 to 700 nm, and not infrared or UV? That can be explained because we do not need it. Our range has been hypothesized to be related to foraging behaviors and our visual system is particularly sensitive in the frequency range of the coloring of (ripe) fruits, which is thought to have been of great benefit to our hominid ancestors (Osorio & Vorobyev, 1996).

Why then do animals extend their vision into UV? Many fish, amphibian, reptilian, avian, and some mammalian species use UV vision. Many birds can identify UV-reflected nectar and berries, and UV-reflecting plumages in birds, and scales in fishes are used for recognition (Shi & Yokoyama, 2003). Moreover, some arthropod species are know to use UV vision to reduce light-reflection distortions under water, such as in the mantis shrimp that features 12 photoreceptor types (as opposed to four in humans) (Thoen et al., 2014).

Why then do animals extend their dynamic range into the infrared? A notable beneficial effect of perceiving infrared is the detection of body heat. The generation of heat is accompanied by the generation of infrared light. The detection of this emitted light is highly useful for nocturnal predators, like the rattle snake (Hartline & Newman, 1982).

- Hartline & Newman, Sci Am (1982); 246(3): 116-27
- Osorio & Vorobyev, Proc Roc Soc B (1996); 263(1370)
- Shi & Yokoyama, PNAS (2003); 100(142003): 8308-13
- Thoen et al., Science (2014); 343(6169): 411-3

Further Reading
1. Is our color vision calibrated to sky, vegetation, and blood?
2. Is there a physical reason for colors to be located in a very narrow band of the EM spectrum?

  • $\begingroup$ Not that you said it was, but I'm not sure if snakes detecting IR light can be classified as vision. Anyways, nice answer +1 $\endgroup$ – canadianer Jun 19 '15 at 0:52
  • $\begingroup$ @canadianer - I classify vision as light perception. People classified as having ultra-low vision (near-blindness) sometimes have not much more than bare light perception (light on or off). $\endgroup$ – AliceD Jun 19 '15 at 0:57
  • $\begingroup$ It's largely a semantics issue I think and I don't really know why I even brought it up ;) A plant can perceive light, thought I wouldn't say it has vision. $\endgroup$ – canadianer Jun 19 '15 at 1:04
  • $\begingroup$ @canadianer - plus I never said that snakes had IR vision in my answer in the first place lol. I used perception $\endgroup$ – AliceD Jun 19 '15 at 1:05

Most of the light from the sun doesn't actually reach the earth's surface due to the atmosphere.

enter image description here


So the light reaching earth includes near-UV, visible, near-IR and a band of radio waves. Seeing any other part of the spectrum would be impossible since it doesn't reach earth.

You asked why we only see in the visible light range; this is due to evolution. Birds, among other animals, can see UV light. In fact, all vertebrates have the potential for near-UV vision. Humans, as vertebrates, also have UV-sensitive photoreceptors. However, our lens is opaque to UV light:

enter image description here

[from Clinical Ocular Anatomy and Physiology via this website]

Just speculating, it would seem that the ability to see near-UV is an ancestral condition which we lost at some point; either it provided no significant advantage (neutral regression) or there was some advantage provided by a UV-opaque lens, either directly (such as protection from UVB light) or indirectly (through pleiotropic antagonism).

On the other hand, human photoreceptors cannot detect IR light. Again, this is a product of evolution. These researchers hypothesize that the longer the wavelength of light detected, the more noise is produced. This noise is due to activation of the pigment molecule by heat. Or, it could just be that never happened. An IR-sensitive photoreceptor might be possible, but evolution doesn't lead to perfect adaptation. In other words, there isn't necessarily a reason why.

As for radio waves, they are too low energy to interact appreciably with matter, at least as far as vision is concerned.

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    $\begingroup$ Great explanation of the physics here. +1 Gimme that Sportsmanship badge ;) $\endgroup$ – AliceD Jun 19 '15 at 0:54
  • $\begingroup$ Most photoreceptors, the way they work, cannot absorb IR because IR is too low in energy to cause electronic transitions. Regarding false alarms; yes heat can generate false alarms but that doesn't mean noisy (in fact IR scatterance is much less. Rayleigh scattering reduces with wavelength). I would reason that diurnal habit and temperate habitat of humans does makes IR sensing not very useful. $\endgroup$ – WYSIWYG Jun 22 '15 at 10:57
  • $\begingroup$ While it is true humans do have some marginal sensitively to UV (basically spillover) we have no UV focused receptors like many other animals. this is true of Mammals in general so the group as a whole seems ot have have lost UV sensitivity. A major problem with infrared is water is not very transparent to infrared, and what are eyes made of. this is why pit vipers have open air filled pits for infrared detection. $\endgroup$ – John Aug 15 '20 at 4:52

Its not really

That is only the human visible spectrum.

Humans actually have a reduced spectrum compared to many animals. Mammals in particular have a reduced spectrum compared ot non-mammals. Reptiles and birds have 4 color sensitive cell types (cones) and can see into the ultraviolet. Many invertebrates can see an even wider spectrum. Mammals lost two of these cells. Modern mammals are descended from early mammals who were nocturnal Thus color vision was less useful. Primates evolved a third cone, (a mutant variant of one of the two they had before) Primates did this because many are frugivores and color is excellent for determining when fruit is ripe.

enter image description here


This is my speculation, but there are no or few organic chemicals which can absorb radio waves having longer wave lengths. Eyes sense lights by absorbing light with organic chemicals, but to sense longer wave length, eyes might need more sophisticated devices like tuners.

In addition, less than 100nm radio waves from the outer space are absorbed by the atmosphere, so even if eyes could sense such radio waves, you don't see any signals on the earth.

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    $\begingroup$ This answer holds great potential, especially the radio wave part. Elaborating on that may get you your deserved upvotes. $\endgroup$ – AliceD Jun 19 '15 at 1:18

"Visible light" is the wavelength of the light that we can see, if human beings could see UV or IR, those wavelengths had been included in the "visible light" definition. Now, our Sun is brightest in yellow-green light, which (you guess?) is right in the midle of the "visible light" spectrum. So the only remaining question is: "Why can we only see that short spectrum?" The answer is EVOLUTION. The same reason that made us to have only 5 fingers in our hands, only 2 eyes and only two kidneys. The bandwith of the "visible spectrum" is enough to make the human beings (as specie) survive.

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    $\begingroup$ Every question can be answered by 'this is EVOLUTION'. I do acknowledge the inherent problematic nature of the why questions, but they nevertheless form an interesting class of questions in the light of speciation. Further, the capitalization should be used with caution as it implies yelling at OP. It is a legitimate question. $\endgroup$ – AliceD Jun 19 '15 at 1:08
  • $\begingroup$ it is also a bad answer using evolution, since there are some pretty solid evolutionary artifacts that make human vision very limited. $\endgroup$ – John Aug 15 '20 at 4:40

These wavelengths happen to satisfy two conditions.

First, shorter wavelength photons (EM radiation) tend to be dangerous for biology. Even UV light (<350nm) is already can damage DNA. That is due to the fact that bond energies in biological molecules tend to have values close to energy of short-wavelength EM radiation. These is why high-energy particles called ionizing radiation. For reference, see this paper. ionization energy for DNA estimated in 4-5eV range, which is 300-250nm.

Secondly, on low-energy hand of spectrum, water absorbs a lot of IR radiation: Water absorption spectrum

So, as you can see, what we see aka visible spectrum is sitting nicely in valley of water absorption but does not extend to area of harsh UV light that will damage chemicals of your body.

Now, why is that so? Because atoms have these masses and electrons around them have these energies. So bonds and molecular interactions have these values.

(In radio range of frequencies you, opposite to UV, have not enough energy to elicit any significant molecular changes, which is why you can't perceive WiFi)

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    $\begingroup$ The reasoning that UV shouldn't be visible because it is dangerous makes no sense. Along that reasoning the visual system should be filtering out predators, guys with guns etc. as well. Moreover, there are classes of critters that sense UV and use it for meaningful purposes. $\endgroup$ – AliceD Jun 18 '15 at 23:56
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    $\begingroup$ Essentially none of the UV light that reaches the surface is ionizing. Lower wavelength UVB light can damage DNA, but this is hitting our body and eye whether we can see it or not. About 95% of UV light that reaches the surface is non-damaging UVA. $\endgroup$ – canadianer Jun 19 '15 at 0:49
  • $\begingroup$ @AliceD My reasoning is following: UV is dangerous hence it is/should be filtered before it reaches anything sensitive. Animals don't really need retinal cancer. $\endgroup$ – aaaaa says reinstate Monica Jun 19 '15 at 0:59
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    $\begingroup$ Reasoning is one thing, claiming it to be so is another, especially since many species actually use UV. Oxygen is highly toxic. Should we therefore neutralize it before we breath? $\endgroup$ – AliceD Jun 19 '15 at 1:00

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