Q19 of 216 Page 469

i. A student suffering from myopia is not able to see distinctly the objects paced beyond 5m. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams, explain

(a) Why the student is unable to see distinctly the objects placed beyond 5 m from his eyes.


(b) The type of the corrective lens used to restore proper vision and how this defect is corrected by the use of this lens.


ii. If, in this case, the numerical value of the focal length of the corrective lens is 5 m, find the power of the lens as per the new Cartesian sign convention.

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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