Question

A person wears glasses of power − 2.5 D. Is the person farsighted or nearsighted? What is the far point of person without the glasses?

Answer 1

The person must be near-sighted because the person wears glasses of power –2.5 D.
For a near-sighted person, u = ∞.
Focal length, f = $\frac{1}{\mathrm{Power}}$ = $\frac{1}{\left(-2.5\right)}$ = − 0.4 m = − 40 cm
The lens formula is given by
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
Putting the values, we get:
$$\begin{gathered} \frac{1}{v}=\frac{1}{\infty }+\frac{1}{\left(-40\right)} \\ \Rightarrow v=-40 \end{gathered}$$
So, the far point of the near-sighted person is 40 cm.