Question

(a) A student suffering from myopia is not able to see distinctly the objects placed beyond 5m. List two possible reasons due to which this defect of vision may have arisen. With the help of a ray diagram, explain.
(i) Why the student is unable to see distinctly the objects placed beyond 5 m from his eyes.
(ii) Which type of corrective lens is 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 1

(a) Two possible reason due to which the 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 eyes.
(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 in the figure below.



Given, focal length, f = 5 m.
Since a concave lens is used, f = -5 m.
Let power of the lens be 'P'.
We know,
$P=\frac{1}{f}$
$\therefore$ $P=-\frac{1}{5}=-0.2D$