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How human eye works

  1. Jul 22, 2015 #1
    I can't understand how eye actually works. From what I understand when I look at 2 objects of the same size The object that is further is smaller in the final image because light rays reflected from it take smaller surface at the retina (Here B is smaller than A because rays take less surface at retina).
    When I watch how accomodation works:

    it seems it changes the shape of lens so less part of retina and fovea (most sensitive part of retina if I am correct) is covered by light rays. So why doesn't it change the size of the final image we see and instead change sharpness of image ?​
  2. jcsd
  3. Jul 22, 2015 #2


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    Perception of size is more complicated than that. For example, if you move your head closer to an object, to some extent it appears bigger. But your vision system also compensates to some degree and makes it appear closer. This is because your vision is taking place to some extent in the nerves and brain. The size of the image on the retina changes, that is part of it. But also the relative angle for the two eyes changes. And some other stuff.

    Size is also judged, quite automatically and without you thinking about it, by such things as relative size of things you (think you) know the size of, what is in front of what, how fast they are moving, etc. This process can be fooled sometimes. For example, this is why very large planes like a 747 seem to not be moving that quickly, while much smaller planes like a Cesna seem to be moving much faster.
  4. Jul 22, 2015 #3
    Also this is why the Sun and Moon seem bigger when near the horizon.
    They are not actually producing a bigger image in the retina, this is to do with how the brain is interpreting the image.
    They just SEEM bigger because of more reference points being in the field of view.
  5. Jul 22, 2015 #4


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    The primary effect is that, when two objects of the same size are located at different differences, the closer one takes up a large angular size. This is a geometric effect. Like your picture shows, the rays from the corners of the blue box have a much larger angle between them compared to the rays from the green box. This directly translates to the light from the blue box falling over a larger area of your retina than light from the green box.

    First, it's important to understand that it is the cornea that is responsible for most of the focusing of the light, not the lens. This is why you can't see clearly underwater. The refractive index of water is much closer to the refractive index of your cornea than air is, so the cornea is much less effective at focusing the light, leading to a blurry image that your lens cannot compensate for.

    The lens is like the fine-focus knob on my telescope. It brings objects into focus by making small adjustments to the light path to very slightly bending the light. The changes to the size of the image are much too small to notice, especially considering that your vision isn't very steady and you're trying to compare blurred vs sharp objects.
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