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A person got his eyes tested. The optician's prescription for the spectacles reads:Left eye: − 3.00 D Right eye: − 3.50 DThe person is having a defect of vision called:(a) presbyopia (b) myopia(c) astigmatism (d) hypermetropia
Explanation
Here, the lens power prescribed by the optician for both the eye has a minus sign $(-)$ which implies that it's a concave lens.
We know that a concave lens is used to correct the defect of vision called near-sightedness or myopia in which a person can't see the distant object clearly (appears blurred), though can see the nearby objects clearly.
Therefore, the child needs concave lenses (or, diverging lenses) in his spectacles, which first diverges the parallel light rays coming from the distant object to form a virtual image at the eye's far point. Then the eye lens easily focuses the rays from the eye's far point to form a clear image on the retina.
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 far point $F$, as the far point of a myopic eye is less than infinity) at the far point of the myopic eye.
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