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Your friend can read a book perfectly well but cannot read the writing on blackboard unless she sits on the front row in class.(a) Is she short-sighted or long-sighted?(b) What type of lenses-converging or perging-would an optician prescribe for her?
(a) She is short-sighted.
(b) An optician would prescribe diverging lenses (concave lens).
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.
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