Question

(a) A student suffering from myopia is not able to see distinctly the objects placed beyond `5 m`. Two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams, explain :
(i) Why the student is unable to see distinctly the objects placed beyond `5 m` from his eyes.
(ii) The type of corrective lens used to restore proper vision and how this defect is corrected by the use of this lens.
(b) If in this case, numerical value of focal length of corrective lens is `5 m`, find the power of the lens as per the new cartesion sign convention.

Answer 1

(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.
.