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Old Jun 6th 2009, 07:45 PM   #1
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Explain how the human eye is able to view objects both from a distance and from up close. In this context explain near-sightedness and far-sightedness and explain how glasses help near and far-sighted people see.
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Old Jun 6th 2009, 08:02 PM   #2
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The human eye has a lens. According if you're seeing a far or near object, there are muscles pressing the lens so that the light entering the eye always reaches the retina in a point. So the person sees well.
Unfortunately many people suffer from eye anomaly, like myopia, hyperopia, etc.
I suggest you to read http://en.wikipedia.org/wiki/Lens_(anatomy) for the rest of your questions.
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Old Jun 7th 2009, 10:03 PM   #3
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As explained by arbolis, the muscles contract or expand and thus change the focal length of the lens so that the image falls on the retina. There is a feedback mechanism and the brain adjusts the focal length till the image is clear. But when there is a problem, this cant be done. We have

1/v +1/u = 1/f where u is the lens-obect distance, v is the lens-image distance, and f is the focal length.

For people with far sight or near sight, lenses are used in spectacles, contact lens etc. such that the combination of this lens and the eye lens has the correct focal length to get a sharp image on the retina.
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