A) A concave lens of focal length - 1.4 m and power + 0.71 D.
B) A convex lens of focal length 1.4 m and power + 0.71 D.
C) A concave lens of focal length - 1.4 m and power - 0.71 D.
D) A convex lens of focal length - 1.4 m and power - 0.71 D
Correct Answer: C
Solution :
In normal eye, the far point is infinity. The lens used should be such that an object at infinity forms virtual image at 1.4 m. \[v=-1.4\,m,\,u=-\infty ,\,f=?\] \[\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\] \[\frac{1}{f}=\frac{1}{1.4}-\frac{1}{-\infty }=-\frac{1}{1.4}\] \[\therefore \]\[f=-1.4\,m\] Power, \[P=\frac{1}{f}=\frac{1}{-1.4}=-0.71\,D\] A concave or divergent lens of focal length m and power - 0.71 D should be used to restore proper vision.
You need to login to perform this action.
You will be redirected in
3 sec
