There are some possible reason due to this defect of vision may have arisen:
(a) Elongation of eyeball
(b) Excessive curvature of the lens.
(a) Student unable to see distinctly the objects placed beyond 5m from his eyes because student suffering with myopia. A myopic eye can't see distinct object clearly but able to see nearby object clearly.
(b) Convex lens of suitable focal length is used to restore proper vision. After using convex, image formed at retina. While before it, image was formed near the retina. As shown in figure it is clear how the lens correct this type of defect.
ii. Focal length of convex lens is positive
So, f = +5m
The dioptre is the unit of measure for the refractive power of a lens. The power of a lens is defined as the reciprocal
of its focal length in meters, or D = 1/f, where D is the power in dioptres and f is the focal length in meters.
Now, Power can be calculated as:
Hence, power is +0.2 dioptre
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