Question

The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

Answer 1

The person is suffering from an eye defect called myopia. In this defect, the image is formed in front of the retina. Hence, a concave lens is used to correct this defect of vision.
Object distance, u = infinity = \(∞\) 
Image distance, v = −80 cm
Focal length = f 
According to the lens formula,
\(\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\)
\(\frac{-1}{80}-\frac{1}{∞}=\frac{1}{f}\)
\(\frac{1}{f}=-\frac{1}{80}\)
\(⇒f=-80\) cm=\(-0.8\) m

We know,
\(\text{Power (P)} = \frac{1}{f}\)
P=\(\frac{1}{-0.8}\)=\(-1.25\ \text{D}\)
A concave lens of power \(-1.25\ \text{D}\) is required by the person to correct his defect.