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Is our Vision System sensitive to Wavelengths or Frequencies?

  1. Apr 22, 2014 #1
    Our Vision System should be sensitive to one specific term, they do not mean the same, one is related with geometry factors (wavelength) while the other is related with beating stimuli (pulse)

    The frequency of light (f) is constant from media to media, the same as it is in vacuum, wavelength is modified by the refractive index of each media, the same happens for the speed of light

    Light into the eyeball travels through the Vitreous Humour (VH) until reaching photoreceptors distributed in the retina, considering a VH refractive index of n = 1.34, and being speed of light in vacuum Co (about 300,000 Km/sec), light wavelength (L) can be calculated (for each frequency) as :

    L = Cv / f = 1.34 Co / f

    It sounds like: Which came first, the chicken or the egg?

    A priori, seems to be equivalent to say that our vision system is sensitive to frequencies or wavelengths, because they are related into our eye through an almost-constant factor (the VH refractive index)

    But, Does aging alter the VH refractive index, causing hue perception shifting? If it doesn’t, our vision system could be sensitive to frequency, not to wavelength (I don’t know the answer)

    Should I consider another key point as Energy transport and absorption?
    In the retina an opsin molecule absorbes a photon and transmits a signal to the photoreceptor cells, the energy of each photon (quantum of light) is absolutely determined by its frequency, not the wavelength

    Is it the opsin capacity to absorbe a photon and transmit a proportional signal to the photoreceptors that decides the question? If it is, this mechanism is function of the frequency, not the wavelength

    Please, could you help me with this dilemma?. Thanks
  2. jcsd
  3. Apr 22, 2014 #2

    Simon Bridge

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    Welcome to PF;
    Why? Biology is capable of producing structures with multiple sensitivities. Real life is not tidily packaged - you'll have been noticing this.

    Note: visual systems are a bit more than light-detection - they include whatever process goes on in the mind to generate the experience of vision. This is part of the mind-body problem - as yet, unsolved. You seem to be asking about how biological eyes act as photodetectors.

    In physics: As far as light is concerned, they are both expressions of energy. You have noticed that the biological photo-responsive systems tend to involve some form of photomultiplication - this is sensitive to energy.
  4. Apr 23, 2014 #3

    Thanks for answering ...

    Yes, I am asking about how our eyes act at the first stage of vision, when light enters the layered structure of retina tissue, before reaching thalamus, specifically in the photopic region where cones responsivity act to produce proximal stimulus

    In physics: As far as light is concerned, they are both expressions of energy. You have noticed that the biological photo-responsive systems tend to involve some form of photomultiplication - this is sensitive to energy.

    While the photon is not absorbed, its energy is function of frequency, not wavelength, it doesn't matter that both terms are related by a equation, Am I wrong in this basic concept?

    Yes, mechanisms like light & dark adaptation, receptors gain control : are sensitive to energy, this is the cause I think that at this stage what matters is frequency, not wavelength

    Away from the eye, where the distal stimulus happens, I could have involved in a clear underwater vision (n = 1.33) or in the air or vacuum (n = 1), or the object I'm looking at could be inmersed in a glass (n = 1.5) full of clear water (n =1) and the colors I see are the same, they depend on the involved frequencies, not the wavelengths

    In the VH before the photon is absorbed the frequency has not been altered by the HV refractive index, while the wavelength yes, if in the first stage, what matters is energy, should be Frequency the key factor …

  5. Apr 23, 2014 #4

    Simon Bridge

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  6. Apr 24, 2014 #5
    wavelength = velocity ( usually the propagation speed of light in the media) divided by frequency
    how can you separate the two?
    asking whether the energy sensitive compounds in the rods and codes of the retina are effected by the wavelength or the frequency seems to me to the same thing
  7. Apr 24, 2014 #6

    Simon Bridge

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    We may as well ask if the way a boat bobs up and down in the water is due to the wavelengths passing or the frequency? Maybe it's responding to the angular velocity of the wave-phasor?

    For light there is less distinction - since wavelength and frequency are properly part of the wave model of light and photons are part of the particle model. DanielMB has started to notice that the wave theory he has been taught has some inconsistencies in it - i.e. it does not mesh that well with geometric waves. This is why we use quantum mechanics. Here, the wavelength of a photon is understood as an intrinsic local property rather than something that is spread out over space.

    note: individual photons travel at c between interactions - the varying speed with media is like the varying mass of the electron in semi-conductors: it allows us to pretend the media is some homogeneous region of space.
  8. Apr 25, 2014 #7
    OK, I know I'm wrong, but I can't understand where ...

    I know that the phenomena involves wavelengths, frequencies, momentum, etc., but what I'm for asking for (at the beginning) is about Energy Transport of light, if Energy is directly proportional to frequency in a photon (suppose just a photon), to know Photon Energy I need only Frequency, if I know wavelength I will need more information to solve the equation, then if it is absorbed in the retina, absorption reactions in the tissue seems to be related to frequency, despite of non-linear responsivities, averaging mechanisms, light adaptation, etc.

    Note : I'm not trying to discard wavelengths, is just that I do not need them in this example

    OK, I know wavelength is conceived as a property, but I need it to get the photon absorbed energy?

    This point is more conflictive (for me), are photons traveling at c between interactions? and all is happening as in black box : it seems to travel with a less speed in the media? lots of discussion about that among physicists, please could you cite a paper or book discussing this point

    Thanks for your patience ...
  9. Apr 25, 2014 #8

    Simon Bridge

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    If a bullet hits a target - does the target experience the wavelength or frequency of the bullet?

    Only because you are used to a particular set of units and tables which separate planks constant from the speed of light. That's just an artifact of the way we do science - nothing to do with Nature.

    It is sometimes mathematically more convenient to use frequency rather than energy - yes.

    But you introduced wavelengths post #1.
    The answer you got is that there is no physical reason to attribute the reactions you are noticing to frequency or wavelength in the manner that you described in post #1.

    It is, however, often more convenient to use frequencies than wavelengths when describing the energy of the light. Not always though. Sometimes it is more convenient to use the wavelength.

    As far as the physical mechanism of the eye is concerned, electrons gain energy from interacting with the incoming light. It is fundamentally an energy interaction.

    The energy of the absorbed photon is determined by the energy states in the molecule/mechanism doing the absorbing.


    It is a common point in most any quantum-mechanics lecture series.

    The QED formulation, for example, has photons travelling at c between interactions.
    To help you understand particle theory of light, I'd recommend sitting through these lectures:
    ... there's lots of them but worth it.

    A kind of executive summary.

    If you want to think of photodetection in the eye as having more to do with frequency than wavelength, then you are free to do so. You are not wrong. But if you ask a physicist which it is you'll get the energy answer.
    Your question has been answered.
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