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Normally our eye is able to see any object clearly between (adults) 25 cm and infinity. The distance 25 cm is called the near point. The far point is the distance up to which we can see clearly(normally infinity). The eye muscles focus (by contracting and expanding themselves) the incoming light rays so that they they converge on the retina. Normal focal length of eye lens is 2.5 cm.

When the muscles become weak and are only able to focus light rays in front of retina (focal length of eye lens < 2.5cm), for light rays coming from far away, we call it Myopia. That means the eye lens is converging too much. Such a person may be able to see the nearby objects alright. So its Short sightedness. We need a diverging lens (Concave) to make the rays converge later at a more distant point.

A person may be able to see the objects nearby clearly. But the eye muscles are able to focus light rays only behind the retina. The converging power has become less and focal length is > 2.5 cm. This defect is called HyperMetropia. We need to wear a converging lens to focus rays on to retina. This is called farsightedness or long sightedness.

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