(a) Two possible reasons due to which myopia are
(i) increase in the length of eye ball
(ii) decrease in focal length of eye lens
The student in unable to see distincly the objects placed beyond `5 m` because a blurred image of such object is formed on the retina, (Fig. 2.5)(b).
The corrective lens used is a concave lens of focal length `5 m`. This would form the image of the object on the retina as shown in (c).
(b) When ` f= -5 m`
`P = (1)/(f) = (1)/(-5) D = -0.2 D`
Negative sign is for concave lens.
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