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Photons Impact on the Visual Process

  1. Jun 12, 2013 #1
    My goal is to build a software program that simulates the function of the eye. As I (barely) understand things, a single photon can bounce off an object and enter my eye.

    Q1 - Does the photon determine the color? If not, what determines the color of that single wave/particle?

    Q2 - How many photoreceptors does a single photon influence? And to what degree?

    Where I'm at in my understanding: I believe we can calculate how many photons a certain light source gives off, thus we can determine how many photons can enter through the pupil knowing the necessary variables (distance, etc.). But imagine we have a light source that emits just one photon in the perfect trajectory to bounce off a flat wall and enter my eye. How many photoreceptors would that single photon impact/influence? In other words, the dreaded question is: what size (how big) should I calculate a photon to be?

    Q3 - Won't I also need to determine the exact path and angle through the pupil in order to know WHICH photoreceptor(s) are impacted by a single photon?


    I'm not looking for a science lesson here. I realize a photon can't have size, but at the same time it does have measurable properties. Perhaps my terminology is off and if we replaced "photon" with "light ray" you could provide a better answer for me. I use photon because the energy level can be quantified and that energy should help me determine if the opsin is sufficiently excited (cones need more, for example).

    I guess I'm hoping for some "in the ballpark" guesses. Or an attempt to have me look at the problem for a different direction.

    In the end, I need to the guidance that will result in a software simulation of the operation of phototransduction.

    Any thoughts?

  2. jcsd
  3. Jun 12, 2013 #2


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    Why don't you look in Wikipedia? They answer a number of your questions.
    It's the other way around. According to Wikipedia, "Rods are extremely sensitive, and can be triggered by as few as 6 photons. Cones require significantly brighter light (i.e., a larger numbers of photons)"
  4. Jun 12, 2013 #3
    Thanks for the reply Bill, but Wiki is referring to how many photons must be absorbed by a single rod photoreceptor in order to create an impulse. However, a single photon may actually influence X number of receptors and that is the number I am after. And FYI - the latest studies reveal that a single photon can trigger a rod photoreceptor - thus highlighting a problem with Wiki.

    I've been to Wiki and it does not answer my question of how many photoreceptors does a single photon influence?

    Nor has there been an attempt to answer my other questions.

    Let me know if there is more information that I can provide in order for you to assist me.
  5. Jun 12, 2013 #4


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    All right then, purely from the physics standpoint, a photon is a quantum mechanical object, and no matter how "wide" it is, one photon cannot strike six objects. When a photon is absorbed, it can only be absorbed at a single point. One photon can therefore influence at most one photoreceptor.
  6. Jun 12, 2013 #5
    Woah, I never thought of that before. Thanks!

    Follow-up...do photons move on the same paths as light? I mean do they scatter in all directions equally - all things being considered equal?

    And what about color...do photons impact the color? In other words if Particle A strikes a single photoreceptor won't the receptor care about what kind of photon color is striking it? Or is that part of that paradox of sometimes particle, sometimes wave?

  7. Jun 12, 2013 #6
    Actually, if you want to go into the details, it is not the Rods or Cones themselves that detect the light, but macromolecules embeded in the membranes of Rod cells and Cone cells called Rhodopsin. (http://en.wikipedia.org/wiki/Rhodopsin). The detailed 3D atomic structure of the molecule has been determined by X-ray crystallography and aspects of the mechanism by which Rhodopsin detects light are known. You can find a somewhat simplified description at http://tinyurl.com/pufffza [Broken]
    Last edited by a moderator: May 6, 2017
  8. Jun 12, 2013 #7
    Thanks Bill and Bill (only on a physics forum site!)

    Let me digest the information. Thanks for the feedback.
  9. Jun 12, 2013 #8
    No paradox here. Rhodopsin is just one type of "opsin", there are a lot of many different "opsins" with ever so slightly different molecular structures which are sensitive to photons of different energies in the visible spectrum. Having just the photons of a given color will excite just those "opsins" sensitive to that energy range. The way that works, each opsin has a cis-retinal molecule bound to it (Retinal is a form of Vitamin A, http://en.wikipedia.org/wiki/Retinal). For the opsin to be activated, the retinal absorbs the photon and transitions from cis to trans configuration, since each opsin binds retinal in a slightly different manner, they will require different amounts of energy to initiate the cis-to-trans conversion.
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