A person cannot see distinctly any object placed beyond 40 cm from his eye. Find the power of lens which will enable him to see distant stars clearly is ?.

Q: A person cannot see distinctly any object placed beyond 40 cm from his eye. Find the power of lens which will enable him to see distant stars clearly is ?.

Sol: The person cannot see objects clearly beyond 0.4 m.
So his far point = 0.4 m distance of object = ∞ . He should use lens which forms image of distant object
(u = ∞)at a distance of 40 cm infront of in it.

$\large – \frac{1}{0.40} – \frac{1}{\infty} = \frac{1}{f}$

$\large p = \frac{1}{f} = – 2.5 D$