The accepted range for the wavelengths of light that the human eye can detect is roughly between 400nm and 700nm. Is it a co-incidence that these wavelengths are identical to those in the Photosynthetically Active Radiation (PAR) range (the wavelength of light used for normal photosynthesis)?

Alternatively is there something special about photons with those energy levels that is leading to stabilising selection in multiple species as diverse as humans and plants?

  • $\begingroup$ The human species (and presumptive many of our close ancestors) have an extraordinary ability to detect shades of green and red. there is a theory behind this saying that we evolved this ability to better distinguish ripe fruits and thus optimizing the foraging. Most other mammals do not in fact have the ability to detect color. P.S. I have no direct reference to this theory, but I have most likely read about it in Campbell & Reece's biology textbook. D.S. $\endgroup$
    – Zewz
    Commented Apr 17, 2012 at 20:54

4 Answers 4


Good question.

If you look at the spectral energy distribution in the accepted answer here, we see that photons with wavelengths less than ~300 nm are absorbed by species such as ozone. Much beyond 750 infrared radiation is largely absorbed by species such as water and carbon dioxide. Therefore the vast majority of solar photons reaching the surface have wavelengths that lie between these two extremes.

Therefore, I would suggest that surface organisms will have adapted to use these wavelengths of light whether it be used in photoreceptors or in photosynthesis since these are the wavelengths available; i.e., organisms have adapted to use these wavelengths of light, rather than these wavelengths being special per se (although in the specific case of photosynthesis there is a photon energy sweet spot).

For example this study suggests that some fungi might actually be able to utilize ionizing radiation in metabolism. This suggests that hypothetical organisms on a world bathed in ionizing radiation may evolve mechanisms to utilize this energy.

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    $\begingroup$ For further reading, this review by Dartnell in 2011 discusses multiple roles cosmic and planetary ionizing radiation may have played in the origin of life. online.liebertpub.com/doi/abs/10.1089/ast.2010.0528 $\endgroup$
    – Ben Haley
    Commented Dec 18, 2012 at 14:58
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    $\begingroup$ Plants and our eyes are evolved to use attainable wavelength to have more Efficiency.The evolved intermediate eyes and plants must have used high wavelength when there was not ozone.If there is a plant which use high wavelength ,Can you give me an example? $\endgroup$
    – user14640
    Commented Feb 27, 2015 at 17:20

The selection you refer in multiple species could be due to a mutual advantage. If fruits absorb visible wavelengths, they can be spotted by other animals and eaten together with the seeds. Seeds can then mature inside the host and, once eliminated with the feces, grow up a new plant in a different place.

This is not only valid for light absorption, but for light emission also: for some fruits, the ripening causes a blue-UV luminescence that can be spotted by some insects.

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    $\begingroup$ If they absorb visible light, they don't reflect it, which would make them harder to see. $\endgroup$ Commented Feb 15, 2012 at 14:56
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    $\begingroup$ They preferentially absorb one or more colors, rendering the others easy to see. For instance, chlorophyl absorb more blue and red light, so you see leaves as green. $\endgroup$ Commented Feb 15, 2012 at 20:22

A rule of thumb in optics is that light interacts with materials that have features with dimensions similar to the wavelength of light. For example, radio waves with large wavelengths interact with large objects like airplanes ,as in the case of radars, and really small wavelengths (x-rays & gamma rays) interact with really small objects like nuclei of atoms. If you take the visible spectrum, light interacts with materials with similar dimensions and/or energies like the C-C, C=O etc that constitute most of the organic compounds. Heck, light with the appropriate wavelength may even interact (or get diffracted in this case) with an organic material that possess many C=O-OH groups spaced with distances similar to the wavelength of light that is shone on it (provided they are regularly spaced and there are lots of them to produce an observable result). Since all organisms are carbon-based the C-C, C=O, C-O, C=_N etc dominate the constituents of the living matter from retina of the human eye to the photosensitive compounds in plants. Hence from the light-matter-interaction perspective all living beings are made up of more or less same materials and this is the reason why plants use the similar wavelength the human eye can detect for photosynthetic processes.

Source: just my intuition

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    $\begingroup$ This makes a lot of sense, and in combination with the "adaption based on wavelengths availability" answer provides the complete answer. $\endgroup$
    – Zuhaib Ali
    Commented Aug 6, 2014 at 14:20

This question is related to the question: Why are some things transparent and others opaque?

Being able to see something requires that it is opaque and that sufficient light illuminates it.

UV and shorter wavelengths are not as prevalent as visible light on earth. The world would appear too dark to see if we used UV and shorter wavelengths. This is because our atmosphere absorbs most high energy light.

Infrared and longer wavelengths of light pass through many objects which would make vision difficult. There's less light here reaching the earth and even less being refracted.

Think about how much our vision relies on indirect light. The frequencies at which most objects are opaque makes those frequencies useful for vision due to the accumulation of refracted light.

Why are many objects opaque in the visible spectrum of light? Longer wavelengths of light have less energy than the valence electrons on most matter. Shorter wavelengths have too much energy, they cause chemical reactions, in addition to not being very prevalent on the surface of the earth.

Electrons are what absorb then remit light and have thresholds based on their chemistry for what they can absorb. No absorption = transparent. Too much energy and chemical reactions start to happen, which may be undesirable, or desirable in the synthesis of vitamin D by UV light.

Plants extract energy for chemical reactions from wavelengths shorter than infrared, which is too weak to drive photosynthesis, and not as abundant as visible light. But also absorb wavelengths longer than ionizing frequencies, which are not very prevalent, and usually cause damage.

Visible light is the spectrum of light which is prevalent enough on earth to see, but is not so energetic it would harm biological systems. The qualities for optimal light frequencies in sight and photosynthesis overlap because they have similar mechanisms for interacting with light. What is this mechanism? The chemistry of carbon based life.


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