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How does glasses work?

  1. Jan 23, 2014 #1

    adjacent

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    Let's talk about Nearsighted people.
    Nearsighted people creates the image of far objects in front of the retina,not on it.
    But produces images of closer objects on the retina
    So we use a concave lens to make the images of the far objects on the retina.Then what will happen to images of near objects?Won't it form behind the retina now because the concave lens increased the focal length of the Eye lens?
     
  2. jcsd
  3. Jan 23, 2014 #2

    rcgldr

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    Nearsighted corretion places virtual images of far objects on the retina. The lens of the eye will need to compensate for closer objects. For glasses that correct for near sighted people, this also means a reduction in size of the image on the retina. For contacts, which are much closer to the lens, the size distortion is much less.
     
  4. Jan 23, 2014 #3

    adjacent

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    Im not talking about the image height.
     
  5. Jan 23, 2014 #4
    Isn't this why they make bifocals? If the prescription works well for distance it is "too strong" for reading. I'm very nearsighted and if I want to see something up close (very close, like 2 or 3 inches) I have to take my glasses off.
     
  6. Jan 23, 2014 #5

    sophiecentaur

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    There are two problems. Young eyes may just have the wrong shape, that will not allow a distant object to focus at all on the retina. They can, however, 'accommodate' for close objects, by muscular action and, once a correcting lens has been placed there, they can see the whole range from infinity to some 'near point'. (The same applies to many young people with 'long sight' - once corrected, they can focus over a range.)
    When you get old, however, the focussing muscles around the lens get stiff and cannot provide such a big range of accommodation. So the bifocals come into play, to cope with a range of object distances. Varifocals are even better, of course, as I found, some time ago, whilst teaching. When I tried bifocals I was either too far away to see students' work, over their shoulders, or so near that I became that pervy teacher leaning too close and breathing down their necks. Varifocals were just the job and I could position myself at a mutually satisfactory distance. Still not a perfect solution when you are trying to work in confined spaces because you have to bend your neck in order to bring close up objects into the lower bit of your vision. Some varifocal prescriptions are really rubbish as they have only a narrow horizontal range. It offends me that I have to pay for the 'best' ones, when they are all made in the same way. Rippoff as usual.
     
  7. Jan 23, 2014 #6

    Drakkith

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    Remember that the lens in your eye changes shape to alter the focus. When you wear eyeglasses for nearsightedness like I do, you have to "focus harder" to see nearby objects with your glasses on than you do with them off because of the divergence introduced by the glasses. If your prescription is strong enough, I'm sure it would make you unable to see very close objects because the increased divergence of the light rays would be more than your eye could compensate for.

    In fact, I just did an experiment. With my glasses on I have to be about 1-2 inches further away from very close objects than I do with my glasses off in order to bring them into focus. By "very close objects" I mean objects about 2-4 inches from my eye.
     
  8. Jan 23, 2014 #7

    adjacent

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    Oh.So that's the reason.Thanks for all the posts

    PS Title should be "How do glasses work?"
    I'm not good at Do does kind of things in all languages
     
    Last edited: Jan 23, 2014
  9. Jan 23, 2014 #8

    jtbell

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    I'm very nearsighted. When I was younger, without my glasses I could focus from maybe 4cm to 30cm. With my glasses I could focus from maybe about 20cm to infinity. (This is from ancient memory, from when I used to do this exercise with students in physics classes.)

    Now, without my glasses, I can focus from about 10cm to 13cm. With the lower part of my bifocal glasses, I can focus from about 30cm to 40cm. With the upper part, I can focus from maybe 100cm to infinity. (I'd have to check my eyeglass prescription and do some calculations to make sure these are consistent.)
     
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