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Working of the human eye

  1. Jul 31, 2013 #1
    I was wondering how a uniform image is formed. If the eye is a point then I can visualize how an image is formed. But the eye isn't exactly a point. So don't light rays from all angles hit all spots on the retina? How is a uniform image formed? For example if we're looking in a plane mirror, we can see objects distinctly because there are only few rays connecting a point of the object to our eye whilst maintaining the laws of uniform reflection. But if the eye is extended then can't light rays from the ceiling and something in front of me reach the same point?
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  3. Jul 31, 2013 #2
    To make my question clearer, consider the image in this link.
    Now, suppose the object is placed at the word 'lens' in the labelling. Now light rays from the object can go from there to any point on the eye lens. How is only one position given to the object by the brain?( Considering it is an object right in front of the eye the position given seems to suggest that light hitting any part of the lens focus to the center of the retina).
  4. Jul 31, 2013 #3


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    If you click on the link 'how the eye works' in the article, your question will be answered.

    If the lens is able to focus the image onto the retina, then we will see a crisp, clear image. However, the lens in the eye will not always be able to focus the image correctly on the retina, as the focal point will be slightly behind or in front of the retina. This will lead to nearsightedness or farsightedness, and the image will become blurry. By adding additional lenses external to the eye (eyeglasses), the focal point can be shifted back onto the retina and the image will no longer be blurry.

    Light from regions outside of the apparent focal plane in front of the eye cannot be focused on the retina and will appear blurry, but certain information can be gleaned by the brain. This is called peripheral vision.

    The brain is responsible for actually turning the optic nerve impulses generated by the retina into the image which we 'see'. A special region of the brain called the visual cortex, which is located at the very back of the cerebrum, forms the image and inverts it, and this is the image one 'sees'.
  5. Jul 31, 2013 #4


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    Consider this: if the human eye could receive input from only a single point of light, our vision would consist of a total of two "pixels". With some saccades we could maybe make more, but nothing near the resolution we actually see.
  6. Aug 1, 2013 #5
    Why two pixels?
  7. Aug 1, 2013 #6
    I still don't get how light from a point source is focused at only one point. Light from the point source can hit all points on the eye lens, and so will be focused at different spots on the retina. Obviously this is wrong as we can see distinctly only because light from a point is focused onto one point. I want to know how light from the point source, hitting all points on the eye lens, can be focused onto one point on the retina.
  8. Aug 1, 2013 #7
    I did a bit of research. Tell me if this is right. Even if light from the point source hits all points of the lens, they ultimately converge somewhere at the other end. That distance depends on the material, curvature of the lens etc. and the rest is up to the brain. Is my basic idea right now?
  9. Aug 1, 2013 #8


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    A point source produces light in all directions, so you can draw concentric spheres around it and light hits the inside surface of the whole sphere. Point sources do not create just a point of light rays in one direction.

    Which end you mean by other end, the camera end or the source end? At the camera end, they don't converge to a single point. Similar to the retina, there's a distributed area of sensors. The source is (approximately) a plane; a cross section in 3D space that you're looking at so it's not a single point either.
  10. Aug 1, 2013 #9

    Andy Resnick

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    Maybe it would help to work 'backwards' from the retina- for any spot on the retina, there is a cone of light that will pass through the lens and pupil of the eye. In terms of geometrical optics, this cone of light will converge onto a single object point.

    Different points on the retina correspond with different cones of light, all with about the same cone angle but oriented in different directions and converging on different object points.

    Conceptually replacing the retina with a CCD is fine for analyzing the optical system (except the retina is curved, not flat) but obscures many essential features of 'seeing'- the retina is part of your brain, and the image formed on the retina is very different than what results after processing.
  11. Aug 1, 2013 #10


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  12. Aug 4, 2013 #11
    Thanks to everyone. I understand now.
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