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

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Solution

A lens which can bring an object at infinity to 40 cm is required here.
So $u = \infty$ and $v = - 40 c m$
we know $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
$\frac{1}{f} = \frac{1}{- 40} - \frac{1}{\infty}$
So $f = - 40 c m = - 0.4 m$
Power of lens $P = \frac{1}{f} = 1 / - 0.4 = - 2.5 D$

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

A lens which can bring an object at infinity to 40 cm is required here.So u=∞ and v=−40cmwe know 1f=1v−1u1f=1−40−1∞So f=−40cm=−0.4mPower of lens P=1f=1/−0.4=−2.5D

A lens which can bring an object at infinity to 40 cm is required here.
So $u = \infty$ and $v = - 40 c m$
we know $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
$\frac{1}{f} = \frac{1}{- 40} - \frac{1}{\infty}$
So $f = - 40 c m = - 0.4 m$
Power of lens $P = \frac{1}{f} = 1 / - 0.4 = - 2.5 D$