Question

A person wears a pair of spectacles with a concave lens of power -2D. Find the farthest point to which he can see objects distinctly without his spectacles.

• A. 25 cm
• B. 30 cm
• C. 50 cm
• D. 20 cm

Answer 1

The correct option is C 50 cm

In the given case, power = -2D = $\frac{1}{f}$
For normal eye the far point is infinity ($\infty$)
So when u = $\infty$, the corrective lens is forming a image at the (defective)far point.
Applying lens formula,
$\left(\frac{1}{v}\right)-\left(\frac{1}{u}\right)=\frac{1}{f}\Rightarrow \frac{1}{v}-\frac{1}{\infty }=-2\Rightarrow \frac{1}{v}=-2\Rightarrow v=0.5m=50$
So the far point for this person is 50 cm.