How do the lenses used in spectacles correct defects of vision?
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Look at the diagrams.
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 the incoming rays converge on the retina. Normal focal length of eye lens is 2.5 cm. So we are able to see the distant objects in a relaxed way and eye muscles are not stretched.
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 it is called Short sightedness. We need a diverging lens (Concave) to make the rays converge e later at a little more distance ie., on the retina.
A person may not be able to see the objects nearby clearly. That is like objects between 25 cm and 50 cm for example. The reason is that the eye muscles are able to focus light rays only behind the retina. They are not able to adjust focal length of the eyelens. 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.
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 the incoming rays converge on the retina. Normal focal length of eye lens is 2.5 cm. So we are able to see the distant objects in a relaxed way and eye muscles are not stretched.
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 it is called Short sightedness. We need a diverging lens (Concave) to make the rays converge e later at a little more distance ie., on the retina.
A person may not be able to see the objects nearby clearly. That is like objects between 25 cm and 50 cm for example. The reason is that the eye muscles are able to focus light rays only behind the retina. They are not able to adjust focal length of the eyelens. 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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