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Light converging inside the eye

  1. Jul 19, 2014 #1
    Does all the light entering our eyes converge to a single point on the retina (assuming minimal aberration)? If this is true then that means that only a tiny point is exposed to light inside the eye so it will hit and activate a single cell (unless the cells are much smaller than the point of light). I just want to know if that's correct. I doubt that because light is processed by millions of cells working together to produce vision. Perhaps light hits a single or a very small number of cells which in turn activate others by some chemical reaction?

    I'm not interested in the biological details, only the optical ones (whether it focuses to a single point)

    I'm sorry if this all sounds absolutely pointless. I'm just curious about this.

    Thank you!
     
  2. jcsd
  3. Jul 19, 2014 #2

    A.T.

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    Not in general.
     
  4. Jul 19, 2014 #3

    jtbell

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    No. If this were true, then all the light entering a camera lens would converge to a single pixel on the camera's sensor, or to a single point on the film in an antique pre-digital camera.

    Ideally, all the light entering the eye from a single point on the object converges to a single point on the retina. But points at different locations on the object produce point-images at different locations on the retina.
     
  5. Jul 19, 2014 #4
    I see. Does this also hold for convex glass lenses? A common diagram in any textbook discussing geometric optics depicts several rays of light from different points converging to a single point.
     
  6. Jul 19, 2014 #5

    jbriggs444

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    Such diagrams normally show several rays of light from the same point on an object striking the lens at different points and then converging onto the same point on the real image.

    Rays from other points on the object converge at other points on the real image.
     
  7. Jul 19, 2014 #6

    jtbell

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    Can you find an example online, or maybe you have one of the books that I have at hand?

    • Halliday, Resnick & Walker, Fundamentals of Physics, 6th ed.
    • Knight, Jones & Field, College Physics, 2nd ed.
    • Hecht, Optics, 4th ed.
    • Pedrotti, Pedrotti & Pedrotti, Introduction to Optics, 3rd ed.
     
  8. Jul 19, 2014 #7

    Drakkith

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    Yes, and sometimes the rays are depicted as being perfectly parallel before they enter the lens, which may lead to some confusion as in reality these rays are *nearly* parallel, not perfectly parallel, and were emitted by a single point on the object.
     
  9. Jul 19, 2014 #8

    A.T.

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    You mean parallel light rays converging to a single point? For very distant light sources, like a distant star, the rays that enter your eye are almost parallel. So despite orignating from different points on the star, they converge into a single point.
     
  10. Jul 19, 2014 #9

    Drakkith

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    Those rays don't originate from different points on the star, they originate from a single point but the distance to the star makes them very nearly parallel.
     
  11. Jul 19, 2014 #10
    Like this for instance: http://www.sciencelearn.org.nz/var/...ging-lens/685327-1-eng-NZ/Converging-lens.jpg

    But Drakkith has made it clear. These rays look parallel, but are actually coming from the same point. Thank you Drakkith.

    So I've learned that rays originating from different points converge at different points. Surely all these points of coincidence are the same distance away from the lens i.e the focal length (assuming the source of light to be infinitely far away)? Also I wonder if, hypothetically, the rays from different points of an object were actually all parallel, would they then all converge on a single focus?

    Thank you everyone. You've been a great help!
     
  12. Jul 19, 2014 #11

    A.T.

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    They do.

    The distance also makes rays from different points on the star nearly parallel, so they all converge in a single point
     
  13. Jul 19, 2014 #12

    jtbell

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    I would rather say that "they all converge nearly on a single point", or "they converge on points that are very very very close together."
     
  14. Jul 19, 2014 #13

    A.T.

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    Yes, sure. Nearly parallel -> nearly a point. For all practical purposes: We see the star as a point, but the light rays come from all over the huge star.
     
  15. Jul 19, 2014 #14
    This makes sense though it raises another confusion for me. If we look through a telescope, the star is no longer a single point, but a big collection of them. Does a telescope not converge the nearly parallel rays to an approximately single point?
     
  16. Jul 19, 2014 #15

    jtbell

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    Diffraction.
     
  17. Jul 19, 2014 #16

    A.T.

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    Explained nicely:

    https://www.youtube.com/watch?v=Rk2izv-c_ts
     
  18. Jul 19, 2014 #17

    Drakkith

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    To simplify things you can consider all the light from the entire star as being parallel, no matter where it originates from on the star. In reality it is not.

    I'd prefer to keep the effects of diffraction a separate discussion from the oncoming rays, as different optical systems will behave differently when viewing an object. For example, my home telescope cannot make out any stars as anything more than point sources. However, the HST has sufficient resolution to directly image Betelgeuse. In both cases the light is very nearly parallel when it arrives at the aperture. It is only once you start converging the light that the situation changes and you have to account for diffraction.
     
  19. Jul 19, 2014 #18

    Chronos

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    On ground based telescopes stellar disc imaging can be achieved using masking interferometry - re: http://arxiv.org/abs/1302.2722. The first image of a stellar disc other than sol was Betelgeuse, achieved by the Hubble in the UV spectrum [shorter wavelengths increase resolving power]. UV images can only be achieved by space based telescopes due to earth's atmosphere. Using the aforementioned interferometry method, R Doradus, one of the largest known stars, was found to have an angular diameter of about 60 milliarcseconds, re: http://arxiv.org/abs/astro-ph/9701021, making it slightly larger than the apparent angular diameter of Betelgeuse. A milliarcsecond is about the size of a billiard ball at a distance of 5000 kilometers.
     
  20. Jul 19, 2014 #19
    One thing that may be interesting to note is that all images are presented to the brain upside-down. Not only that, but each eye sends half the image to each side of the brain, through the optic chiasm. And, No, "Does all the light entering our eyes converge to a single point on the retina." Ideally the light spreads itself out to cover the foveal region of the retina. People that need glasses use these to optimize the focus on this foveal region.

    So, this was really a lesson for me in my early studies of neurobiology. Your senses are not presented to you like you think a video camera might present them. It actually arrives mostly in a confused and disorganized manner through your dorsal root ganglia and cranial nerves. It is the job of the brain to "construct" from this an image of the world. I try to impress this fact on physicists at my university all the time, try to incorporate not only the data your sensors are giving you, but also try to account for the manner in which your brain recognizes and interprets the data. That sentiment typically falls on deaf ears, though.
     
    Last edited: Jul 19, 2014
  21. Jul 19, 2014 #20

    Drakkith

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    That's not really an accurate description of what happens. Also, this is more of an optics question than a biology question, so the details of how the brain reconstructs the image is probably best left to another thread.
     
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