(a) A student suffering from myopia is not able to see distinctly the objects placed beyond 5 m. List 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 the corrective lens used to restore proper vision and how this defect is corrected by the use of this lens. |
(b) 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. |
Answer:
(a) Two possible reason due to which this defect of vision may have arisen are:
(1) increase in curvature of the lens.
(2) increase in length of the eyeball.
(i) A myopic eye has its far point nearer than infinity. It forms the image of a distant object in front of its retina as shown below:
In the given case, student's far point is 5 m. So, image of the object placed beyond 5 m from his eyes is formed in front of the retina and hence appears blurred. That is why the student is unable to see distinctly the objects placed beyond 5 m from his eye.
(ii) Since a concave lens has an ability to diverge incoming rays, it is used to correct this defect of vision. The image is allowed to form at the retina by using a concave lens of suitable power as shown.
(b) Power, \[P=\frac{1}{f(m)}\]
\[P=-\frac{1}{5}=-0.2\,D\]
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