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Complete the following sentences:(a) A short-sighted person cannot see _________ objects clearly. Short-sightedness can be corrected by using _________ lenses.(b) A long-sighted person cannot see _________ objects clearly. Long-sightedness can be corrected by using _________ lenses.
(a) A short-sighted person cannot see distant objects clearly. Short-sightedness can be corrected by using concave lenses.
(b) A long-sighted person cannot see nearby objects clearly. Long-sightedness can be corrected by using convex lenses.
Explanation
Myopia, also known as near-sightedness or short-sightedness, is a defect of vision in which a person can't see the distant object clearly (appears blurred), though can see the nearby objects clearly. The far point of a myopic eye is less than infinity.
This defect occurs either due to the high converging power of the eye lens, (because of its short focal length). Or, due to the eye-ball being too long, which causes light to focus in front of the retina, instead of directly on the retina.
It is corrected by using spectacles containing concave lenses, which should be of such a focal length (or power) that it produces a virtual image of the distant object (lying at infinity) at the far point of the myopic eye.
Hypermetropia, also known as long-sightedness or far-sightedness, is a defect of vision in which a person can't see the nearby object clearly (appears blurred), though can see the distant objects clearly. The near point of a hypermetropic eye is more than 25 cm away.
This defect occurs either due to the low converging power of the eye lens, (because of its large focal length). Or, due to the eye-ball being too short, which causes light to focus beyond the retina, instead of directly on the retina.
It is corrected by using spectacles containing convex lenses, which should be of such a focal length (or power) that it forms a virtual image of the object (lying at the normal near point N of 25 cm) at the near point 'N' of the hypermetropic eye.
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