Question

A person suffering from myopia cannot see clearly beyond a distance of $1.5\text{m}$. Calculate the power of the lens used to correct this defect.

• A. $-1\text{D}$
• B. $1.5\text{D}$
• C. $0.67\text{D}$
• D. $1.25\text{D}$

Answer 1

The correct option is C $0.67\text{D}$

According to lens equation, we have,

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

Consider an object at infinity. The lens should form a virtual image of this object at $1.5\text{m}$ from the lens.
So, $u=\infty ,v=-1.5\text{m}$

Thus, it should be a divergent lens and its focal length should be $-1.5\text{m}$

So, power, $P=\frac{1}{f}=\frac{1}{-1.5}=-0.67\text{D}$

Hence, $\left(C\right)$ is the correct answer.

$$\begin{array}{c}\text{Why this Question ?}\\ \text{Concept : Myopia can be corrected using divergent lens.}\end{array}$$