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Magnifying Glass and Eye Glasses Comparison

  1. Jan 26, 2013 #1
    Do glasses work in the same way that a magnifying glass does. They take light and focus it onto a point? If a magnifying glass can burn an ant, do eye glasses burn our eyes to a certain degree? Could it just be that eye glass wearers don't stare at the sun?

    I hope you guys understand what I am asking.
    Lets hear what you have to say on this topic.
  2. jcsd
  3. Jan 26, 2013 #2

    Simon Bridge

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    A magnifying glass is a converging lens - it tends to make light rays converge.
    In particular, parallel rays will converge to a point - called the "focus".
    Light scattered from something, passing through the lens, will converge to an image - so the light will appear to the eye as if it were coming from the image location.

    The process of "focussing" often refers to obtaining a sharp image.
    When the image is "out of focus" it is blurry.

    Eye-glasses - to correct long-sightedness - work the same way.
    You can test this very easily if you know someone who is long-sighted.

    When you wear glasses, your eye is well inside the focal length - you'll notice you have to get an ant pretty well in the focus to burn it.

    Light-rays scattered through the lens from a nearby object are diverging quite a bit more than the eyes can handle - the lens makes them diverge less (by converging them) and so makes objects closer than the eye's near-point appear close to the near point.

    The opposite is needed to correct hear-sightedness, so a near-sighted persons glasses are diverging lenses rather than magnifying glasses. They work like solid glass bi-concave lenses in air.
  4. Jan 27, 2013 #3
    That's interesting but it still kind of leaves my question out there. So it appears that to burn the ant, it has to be located where the light converges from the magnifying glass. The same happens with the eye glasses of a far sighted person but why aren't our eyes damaged like an ant who sits at the lights focal point.
  5. Jan 29, 2013 #4

    Simon Bridge

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    The same reason the ant does not get damaged when it is not sitting at the focus. I did say, and you did notice:
    note: "focus" means "fireplace" - you stay out of the fireplace, you don't get burned.

    You would get slightly more damage from light while wearing converging lenses than if you don't (over identical conditions) ... but it is very slight (work out the change in flux between wearing and not wearing) and the wear-and-tear from not wearing correcting lenses when you need them would be much worse.

    The hypothetical extra wear and tear vanishes against the changes in the eye as you get older - just normal biological processes.
  6. Jan 29, 2013 #5
    Even without glasses, you don't look directly at the sun if you want to keep your retina intact.
    Anyway you have a converging lens in your eye. The glasses are just correcting the defects in this lens. If this lens focuses the sun's image on your retina, it may produce permanent damage. So never look directly to the sun.
  7. Jan 29, 2013 #6


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    I am holding a pretty strong pair of reading glasses (convex, image forming lenses) according to the numbers on the frame they are 3.25 (what ever that number refers to). I can find the approximate focal length by forming an image of a distant object. The overhead lights are pretty far away (10xfocal length is essentially at infinity and will form an image at the focal length. They form an image in the range of 15-20cm. So the focal length is about 15cm, when I am wearing them my eye is about 2 cm from the lens, no where near the focused image of a distant object. This is why you don't burn your eye when wearing them.
  8. Jan 29, 2013 #7
    Okay I understand this better now. It was hard for me to understand how glasses worked if our eyes are within the focal length. But they only do part of the job that is converging the light from the image. They help put the light on the right path and the inside of our eye does the rest of the converging... thanks for the input everyone
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