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Eye and vision during low intensity

  1. Feb 21, 2015 #1

    Suraj M

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    This is something that i noticed.i.e., it happen to me, when i went to my teacher to ask him why it happens he actually didn't believe me, i guess maybe because he was a physics teacher!
    Consider a room with\ all the lights switched off, let their be a far off light source which is very weak. So you can't actually see anything clearly but you can make out the boundaries of things around you, faintly.
    What i observed was that when there was a particular object in front of me, or even if the dim far off light source is
    in front of me and i tried to look straight at it i could not see it, but if i moved my line of sight away from the source, i could see it.
    Summary, could not see it when i look right at it but could see it in the peripheral part of my vision, t when i looked at something else.
    why does that happen?
    I thought about this for a few years actually because no one believed me! I hope you guys do!
    I actually came up with a lame excuse, it was that, maybe because when i look straight at a thing i'm sensing it mostly by my fovea which is loaded with cones, i just assumed that as there are so many cones there would be lesser rods, hence bringing down my sensitivity at low intensities of light! Is that right?
    Also try doing this and see if you observe something similar to what i tried to describe above!
     
  2. jcsd
  3. Feb 21, 2015 #2

    phyzguy

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    This is a well-known phenomenon, and your explanation is correct. See this link for a diagram of the rod and cone density in the eye. Astronomers are well aware of this, and often use a technique called "averted vision", where you look slightly away from the object you are trying to see. This enables you to see fainter objects for the reason you described.
     
  4. Feb 21, 2015 #3

    Suraj M

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    Oh! thanks for the link, if you dont mind me adding another question- I've tried the to do the same with a red coloured source, the link you gave says the fovea has red cones. Now though the light is still dim its also red so do rods react to it or cones?
     
  5. Feb 21, 2015 #4

    Pythagorean

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    Only cones detect color, but rods are more sensitive to light in general (which is why they still work in low-light conditions). If you could see red, that means there was enough light to activate your cones. But there are more red cones (not that that necessarily correspond to the perception of red, you'd have to look at the chart*).

    you can see the radial spatial distribution here:
    http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html

    * the response curve chart:
    http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colcon.html#c1
     
    Last edited: Feb 21, 2015
  6. Feb 21, 2015 #5

    Suraj M

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    Pythagorean, if you see the penultimate line in the second link you posted, it says at least 2 types of cones should be triggered for the colour to be sensed, then again how can we see only red?
    and also it means that if i reduce the intensity of the red light even more it would turn white? as the cone does not react at that intensity, only rods would
     
  7. Feb 21, 2015 #6

    Pythagorean

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    The "color" of the cones isn't directly related to the color you perceive. The color you perceive is an abstraction of the combination of which cones are firing and to what degree.

    I don't know what the threshold values are. It's possible that any human light source you use will always have too high of a threshold when it's on and will thus always trigger color perception... but I don't know.
     
  8. Feb 21, 2015 #7

    phyzguy

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    My experience with this has been the following. Go outside (not in the city - someplace dark like in the woods or out in the country) before dawn. You will find that you can see things, but that everything is essentially black and white. As dawn approaches, the overall light intensity increases until eventually you can see things in color. I've found it to be quite sudden - it seems like the colors switch on all at once. However, this may just be my perception. You could probably simulate this in a dark room where you gradually increase the light intensity.
     
  9. Feb 21, 2015 #8

    Evo

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    Are you aware of the 'blind spot"? This may or may not be what you experienced, but it's a fun example.

    https://visionaryeyecare.wordpress.com/2008/08/04/eye-test-find-your-blind-spot-in-each-eye/
     
  10. Feb 21, 2015 #9

    Suraj M

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    Yes Evo i'm aware of it(done that experiment a lot of times :) ) and its not exactly in the line of sight, If i look straight at an object my fovea would recognize it and also i have 2 eyes, so the blind spot shouldn't matter.
     
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