Question

A person suffering from an eyesight defect has a far point at $40cm$ and a near the point at $25cm$. The person uses a lens to see far away object. Find the near point of the person while wearing this lens.

• A. $50cm$
• B. $\frac{200}{13}cm$
• C. $\frac{200}{3}cm$
• D. $25cm$

Answer 1

Answer: (C)

$\frac{200}{3}cm$

Far point $=40cm$

$\Rightarrow$ If object is at $\infty$, it image should form at $40cm$ by lens.

Now,

$\begin{array}{cc}& \frac{1}{40}-0=\frac{1}{f}\\ \Rightarrow & f=40cm.\end{array}$

Near hoint $=25cm$

With lenses on, the image will be formed at:

$\begin{array}{cc}\frac{1}{v}& =\frac{1}{40}-\frac{1}{25}\\ & =\frac{-3}{200}\\ v& =\frac{-200}{3}cm.\end{array}$

$\text{Ans:}\frac{200}{3}cm$