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B Why "light"?

  1. Jun 19, 2017 #1
    I am a bit confused about my conceptual understanding of "light". As always I warn that I have only the simplest naive understanding of physics.

    In thinking about vision, especially color vision, I understand that color as such is not a real property of the external world. Nor for that matter is light and dark. So when we talk about light, we are really talking only about electromagnetic radiation (EMR) in a limited range of frequencies.

    As "light" and color are subjective inner properties assigned by our brains to the interaction between our eyes and EMR, I'm wondering exactly why our eyes/brains have evolved to use the limited range of frequencies that make up visible light.

    My question is simply, are there particular qualities of EMR that differ one from the other depending upon essential properties such as frequency or amplitude? A quick read about the electromagnetic effect suggests so, given that the threshold frequency appears to be visible light (I confess that seems to me a strange coincidence).

    My naive take about EM radiation is that its a sort of linear thing - waves/particles that differ largely only in frequency/wavelength/intensity, but clearly there is a lot more to it? Or do I misunderstand things here completely?
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  3. Jun 19, 2017 #2


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    it's just the way it is .... other creatures can "see" in different wavelengths, a number of them have good IR vision

    not sure what to make of that ??

    about the electromagnetic effect ??

    what is that ?

    the EM spectrum is just that ....a wide range of freq/wavelength ....different animals/insects have "vision" that covers different upper parts of it

    I don't really know what more you expect ?

    We, the science community, have equipment that can detect/receive everything on the E/M spectrum from 0 Hz frequency right up into the sub millimetre wavelength ( gamma waves etc)

  4. Jun 19, 2017 #3
    Sorry, I meant the photoelectric effect. What I'm getting at is that when I think of eyes evolving, I note that generally they seem to work only for a limited range of frequencies - visible light, some IR etc. But what is it about those frequencies that led to eyes evolving. Reading about the photoelectric effect tells me that there are behaviours that are tightly related to frequency, so is there something special about the frequency range of "visible light" that makes it more likely organisms would evolve the capacity to detect it? Why not some other frequency? Why not gamma rays?
  5. Jun 19, 2017 #4


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    You also have to consider the structures required to receive frequencies longer than infrared and more energetic than near UV. To go to what we consider radio waves requires very large receiving structures or molecules compared to what is present today. These have not been developed so that indicates they are very hard or impossible in evolutionary terms.

    Farther past the near UV and into X-rays would require resistance to ionizing radiation. Also those wavelengths are blocked by the atmosphere so would be of limited or no use. It is dark much higher than UV.

    Basically life uses the easy part of what is available.

  6. Jun 19, 2017 #5


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    Any understanding of human and animal visual sensitivity ranges must start with development and progression of what we now call sight and the environments and survival pressures under which those developments progressed and diverged.

    As an example, seeing into the infrared region could be found to be more important than color vision for nocturnal animals who hunt or need avoid to being found in dark environment. Alternatively, for primarily diurnal (daytime) functioning entities, that level of infrared sensitivity might result in the early onset of damage to the retinal cones that provide color differentiation that is beneficial for identification and classifications in the lighted environment.
  7. Jun 20, 2017 #6
    JBA, I guess I didn't specifically mean any species, more why organisms at the beginning evolved detectors for what we call "visible light". Given that without organisms there is no such thing as a lighted environment, it's not immediately obvious to me why detectors for such a specific range of frequencies would evolve.

    But rbelli1 is making the case for some distinctive property of those frequencies and their interaction with the biological structures to be found in organisms - that for example radio waves might need large organic molecules to be detected. Though my question then is, why would that be? Not trying to be obtuse, I just don't know enough. And I don't know what it means to say that it is dark above UV wavelengths?
  8. Jun 20, 2017 #7
    I mean, I guess I could think that maybe the sun emits EMR primarily in wavelengths between 200 and say 1000nm, and so life would have evolved to take advantage of those wavelengths, but a quick Google suggests that the sun emits a wide range of EMR:

    "The Sun emits radiation right across the electromagnetic spectrum, from extremely high-energy X-rays to ultra-long-wavelength radio waves, and everything in-between. The peak of this emission occurs in the visible portion of the spectrum."

    Is it simply that the larger proportion of EMR is in the "visible" wavelengths? Or do wavelengths well outside that band cause damage to the kinds of organs that might evolve to detect EMR?
  9. Jun 20, 2017 #8


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    The intensity of electromagnetic radiation falls sharply after peaking in the green region of the visible spectrum. By the time you get to the highest-energy UV light and beyond the intensity is already well under 1% of the peak. That's why we say that it is essentially "dark" above UV wavelengths. The Sun just doesn't put out much x-ray and gamma radiation (or even UV) compared to the visible and IR light.

    Ultraviolet light crosses the boundary between ionizing and non-ionizing radiation. The lowest energy UV light is relatively harmless, but just beyond that the energy per photon reaches a point where it is large enough to ionize certain molecules important to biology, kicking electrons off of them and breaking them apart. Once you get past the near-UV portion of the spectrum, there just aren't many biologically important molecules capable of detecting this radiation without being ionized and subsequently causing damage to the organism.

    On the other end of the spectrum, IR light is generally of too low energy per photon to generate any useful chemical reactions or induce structural changes to molecules, which means that organisms generally have no way of detecting and using this radiation. At best they can passively absorb it to help warm their bodies.

    Visible light sits in a narrow niche where the energy is high enough to be useful, but low enough to be safe.
  10. Jun 20, 2017 #9


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    It's the range the Sun radiates the most in, so it stands to reason that evolution of diurnal animals would favour detectors in this range.
    (I see you got to the same conclusion while I was writing this response)

    In terms of detection mechanisms themselves, and in relation to your comment on photoelectric effect, visual range EM radiation interacts with molecular-level excitation rather than atomic-level as in the former case. I.e. chemicals in the light-sensitive eye structures can get excited by impinging light of certain wavelengths, leading to neural response. The excitation can take the form of weak bond breaking or vibrational excitation - triggering chemical reactions or isomeric changes.
    See this detailed overview:

    Too long a wavelength and it cannot 'see' the molecules (as a rule of thumb, wavelength must be comparable to nucleus/atom/molecule/antenna size in order to interact with it). Too short a wavelength, and it starts to break stronger bonds, and ionise atoms, causing damage. These are not necessarily unassailable obstacles for evolution of EM detectors, but since the planet is inundated with visible light, there's little advantage in developing them.

    (and I see Drakkith covered most of these points above)
  11. Jun 20, 2017 #10
    Ah, excellent, that all explains it very well. Thanks to all who offered comments, very much appreciated.
  12. Jun 20, 2017 #11


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    Biological Engineering is a fascinating topic.
    One way to look at our 'choice' of spectral range for our vision is in terms of signal to noise ratio. SNR is relevant in all forms of measurement and detection - natural and man-made. Radio comms only works because it is possible to build transmitters with enough power so that the received signal has significantly more power than the other sources of radio frequency power (interference and random noise). We evolved our vision, based on the spectral range of light signals available and the amount of available information that can be extracted. In addition to the spectrum of light from our Sun, there is a 'window' in the atmosphere which lets through the 'visible' range more than other frequencies. (High Signal Levels)
    Why not 'see' with UV? UV photons are energetic and cause damage. That would mean we would need to have evolved 'hardened' sensors for reliable detection of UV. Better to filter out the UV in the optics before it hits the sensors. (personal but reasonable? view). The UV absorption of organic substances means that surface grease etc. produces a more confused image than the optical version.
    "Other reflected-UV applications involve the detection of small amounts of surface contamination. Since UV light tends to be absorbed by organic materials, traces of oil or grease are sometimes detectable on many surfaces, particularly in the deep-UV band (see Fig. 2). It is also possible to distinguish new paint from old in some situations, even when the two types of painted surfaces look identical in the visible band (see Fig. 3)." ( from this link )
    If you Google "UV Imaging" with the Images option, you will see many examples of the confused picture that UV imaging produces, compared with the optical version. I guess it was just not worth evolving UV vision, bearing in mind the cost to the organism. Some short lived insects and flowers came to an arrangement that uses UV images to encourage pollination. Perhaps their sensors work well enough for a year or so.
    Why not use IR? Signal to Noise ratio is very relevant here. IR would be no use in the day because all surfaces tend to heat up to similar temperatures and would change colour and 'look' similar. A great advantage at night but the technology to produce the night time wildlife pictures we get from thermal cameras seems to have evaded even nocturnal creatures. There is the example of snakes which detect prey by its radiated heat but it's a pretty low res system. I'd bet that's due to SNR problems - not least due to the already warm eyes of mammals. The best thermal cameras use cooled detectors, I believe.
    Evolution always tends to find near optimal solutions for living organisms (else they die out). It blindly follows an advantageous path so the 'why' question never gets asked by the 'fittest' which 'survived' better. You can only use dodgy post-hoc rationalisation - same as Historians and Anthropologists.
  13. Jun 20, 2017 #12


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    May have something to do with fact that early development of creatures with effective eyesight took place when the creatures lived mostly in the sea .

    The range of EM frequencies that is not strongly attenuated by the water and at the same time useful for seeing purposes is probably quite restricted .
  14. Jun 20, 2017 #13
    Thanks for this link, that was a very nice clear description. I hadn't realised the process was as multilayered. And thanks sophiecentaur for some further insights on the matter.
  15. Jun 20, 2017 #14


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    According to wikipedia:

    The facial pit actually visualizes thermal radiation using the same optical principles as a pinhole camera, wherein the location of a source of thermal radiation is determined by the location of the radiation on the membrane of the heat pit. However, studies that have visualized the thermal images seen by the facial pit using computer analysis have suggested that the resolution is actually extremely poor. The size of the opening of the pit results in poor resolution of small, warm objects, and coupled with the pit's small size and subsequent poor heat conduction, the image produced is of extremely low resolution and contrast. It is known that some focusing and sharpening of the image occurs in the lateral descending trigeminal tract, and it is possible that the visual and infrared integration that occurs in the tectum may also be used to help sharpen the image. In addition, snakes may deliberately choose ambush sites with low thermal background radiation (colder areas) to maximize the contrast of their warm prey in order to achieve such a high degree of accuracy from their thermal “vision”.

  16. Jun 20, 2017 #15
    Even after reading the full thread I still do not know what you mean with "particular qualities of EMR". There actualy are species that have sensors for properties like polarisation of light or for the orientation of magnetic fields. But this is useful for special conditions only (e.g. polarisation for environments with strong scattering or magnetic sensors for long-distance navigation) and haven't been essential for the evolution of humans.

    If you mean other frequencies than there are at least three limitations. First there is the absorption spectra of water with a minium in the range of visible light. As live evolved in water and our eyes still mainly consist of it, it makes sense to use these frequencies. The second limitation is the resistance against energetic radiation. Chromophors for UV radiadiation will aborb a lot of energy and therefore take damage which needs to be repaired. That is only useful with a corresponding benefit, which seems to be the case for many insects but not for humans. The third limitation is the heat emission of the organism itself - especially of endotherms. It wouldn't make much sense to build a heat cam that shines brighter than the envoronment.
  17. Jun 20, 2017 #16
    DrStupid, I understand the basics of how eyes detect light. It just wasn't obvious to me why detectors evolved that tend to operate within such a narrow range of wavelengths. I naively imagined the early earth's surface bathed in a uniform sea of radiation. It never occurred to me that the obvious factor is the profile of available solar radiation, but as you and others point out there are also certain properties involved, such as low energies at wavelengths in and above IR and ionising effects at those in the UV.
    Last edited: Jun 20, 2017
  18. Jun 20, 2017 #17
    Just as an aside, the sense that I get from various explanations of visual perception is that it's sort of a one to one thing. Receptors respond to light and signals are sent to the brain where the shapes and colors are "rendered" as an image. But like so many other aspects of subjective experience, it seems that what colors people consciously perceive does depend to an extent on whether they've learned to discriminate and name those colors. So in addition to the raw mechanics is the process of learning which colors are what. I think that's fascinating!
  19. Jun 20, 2017 #18


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    I think we have to indulge the OP in this respect. He admits to very little Physics knowledge and asked in a vague way. The answer to that particular question deserves its own (very long) thread. In fact it doesn't lend itself to Q and A at all. The OP really needs to be reading around the subject of EM waves and that would need to involve his picking out texts at a level that he feels comfortable with. Perhaps some high school level texts would be good to start with. After that, the sky's the limit.
  20. Jun 20, 2017 #19


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    Your latest posts show that you are really coming to grips with this subject.
    I think a better word than "image" here would be 'Model' or even 'Map'. We use many more clues than visual ones to assemble our internal model of the world, our sense of place, orientation (within the Country or on Earth) and the way we assemble internal models of things which we never actually see (visually). Sound and touch also contribute (check out the inside of your mouth; you have probably never seen it but you know your way around it 'like the back of your hand').
  21. Jun 20, 2017 #20


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    I think the question is well-suited to PF and I don't think it's particularly vague. The answer involves both properties of EM radiation and properties of biological molecules and sensory organs. This isn't something you can just pick up a book and learn, it's only by having specific knowledge of multiple areas can you put two and two together.

    That depends on what you mean by "perceive". If a society doesn't a word for, say, "turquoise", they can certainly still see that color and discriminate between turquoise and other colors, they just don't have a name for it.

    Also, the explanations above are only explaining how the retinal cells interact with light. Color perception, and visual perception as a whole really, is extremely complicated and involves multiple stages of processing both before and after the signals enter the brain.
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