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Optics and The Eye

  1. Feb 3, 2008 #1
    Hi.

    I'm new to this forum and wondered if anyone can help with this problem I have understanding how vision works.

    In some ray diagrams of the eye it show the rays converging to a point on the retina and on some diagrams it shows them converging before the retina and arriving at the retina inverted.

    The second one makes more sense to me as I imagine that to be the inverted image being formed on the retina. (much the same as a lens forming an image on a piece of paper).

    However the other diagram appears on many website and textbooks too.. Surely the rays of light are not converged into a single point on the retina? How would the retina and optic nerve make sense of this??

    Can anyone help with my mis conceptions?!

    Thanks
     
  2. jcsd
  3. Feb 3, 2008 #2

    Doc Al

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  4. Feb 3, 2008 #3
    Thanks.

    So does that mean the websites that show parrallel rays from infinity converging to a single point on the retina are wrong??
     
  5. Feb 3, 2008 #4

    Doc Al

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    No, but the light from some object is not going to be parallel. Unless it is infinitely far away. (And then you can't see it!) If it is far enough away, it will look like a little dot anyway.
     
  6. Feb 4, 2008 #5

    Andy Resnick

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    Oof- lots of stuff here. It just goes to show that the eye is much more complex than is usually discussed in optics classes.

    A few preliminaries: there are focal planes and pupil planes. Pupil planes are Fourier transforms of focal planes.

    So, for a relaxed eye focused at infinity, an object point corresponds to a plane wave at the pupil plane (typically located near the lens), which is then focused to a point at the retina (neglecting aberrations). When we focus on an object that is closer than infinity, by manipulating the power of the lens within our eye, the image is formed on the retina (for normal or corrected vision). This explains both diagrams.

    To be sure, there is substantial image processing within the retinal layers: edge enhancement, motion detection, local averaging, all kinds of stuff. The actual image projected onto a retina looks horribly aberrated.

    For all this and much more, see "Optics of the Human Eye" (Atchison and Smith)
     
  7. Feb 4, 2008 #6

    jtbell

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    If the second diagram shows light rays converging to a point in the middle of the eyeball, then diverging to hit the retina, that's wrong, at least for an eye that's actually focused on the object. Each individual point on the object produces a bundle of light rays that pass through the lens and are focused to an image point on the retina. The rays diverge as they enter the lens, and the lens converges them towards the image point. Different object points produce different image points. The overall image is inverted, but the inversion effectively happens at the lens, not in the middle of the eyeball.
     
  8. Feb 4, 2008 #7
    Thanks for your help everyone. So you could draw a diagram showing parallel rays coming from different points on a object or many rays coming from a single point on an object in all direction? Is this the same as when you hold a lense up to a window and get a real image formed on a sheet of paper the other side of the lense?
     
  9. Feb 4, 2008 #8

    jtbell

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    Yes, the image formed by the lens of the eye on the retina is a real image just like the image on a sheet of paper in your example.
     
  10. Feb 5, 2008 #9
    Ok, sorry to keep going on about this but i think i've got it.

    Am I right then in saying that this video clip on you tube is incorrect when it talks about pararrel rays entering the eye?

    It shows the rays converging before the retina. Surely that would mean the person is short sighted?

    http://www.youtube.com/watch?v=15P8q35vNHw&feature=related
     
  11. Feb 5, 2008 #10

    Doc Al

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    I'd say that the video clip is misleading when it shows rays converging before the retina. (What I think they are actually showing are principal rays used to determine where the image is focused. That criss-cross point is not the image. Confusing, eh?)

    The image is focused on the retina, except as you noted in the case of nearsightedness (or farsightedness).

    You might find this a more informative discussion of the eye and vision: Image Formation and Detection

    Here's a good picture of how a lens (like your eye) focuses light to form a real (and inverted) image: Real Image Formation
     
  12. Feb 5, 2008 #11
    Ophthalamology students have been known to carefully scrape the back off a sheep's eyeball to see that image.
     
  13. Feb 5, 2008 #12
    Thank you.

    Sorry, last question on this I promise!

    If I hold a converging lens to some writing and move the lens gradually nearer to the page until the writting appears in focus is the distance now between the lens and the paper the focal length?
     
  14. Feb 6, 2008 #13

    Andy Resnick

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    No, becasue of the refractive power (and accomodation ability) of your eye. The quick and dirty method of determining the focal length of a lens is to image a light bulb (ceiling fluorescents are great) onto the floor; the source is effectively at infinity, so the distance from lens to floor is the focal length. YMMV.
     
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