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A man can read the number of a distant bus clearly but he finds difficulty in reading a book.(a) From which defect of the eye is he suffering?(b) What type of spectacle lens should he use to correct the defect?
(a) He is suffering from a defect of the eye called hypermetropia (long-sightedness).
(b) To correct this defect, he should use convex lenses in his spectacles.
Explanation
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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