The least distance of vision of a longsighted person is 60 cm. By using a spectacle lens, this distance is reduced to 12 cm. The power of the lens is

Question

The least distance of vision of a longsighted person is 60 cm. By using a spectacle lens, this distance is reduced to 12 cm. The power of the lens is (A) +5.0 D (B) +(20/3)D (C) -(10/3)D (D) +2.0 D

Tardigrade Admin · Accepted Answer

Here, v=-60 cm , u=-12 cm ∴ By using the relation, (1/f)=(1/v)-(1/u) We have, (1/f)=(1/-60)-(1/-12) ⇒ (1/f)=(1/15) cm or (100/15) m So, the power of lens, P=(100/15)=+(20/3) D

Q. The least distance of vision of a longsighted person is $60 c m$ . By using a spectacle lens, this distance is reduced to $12 c m$ . The power of the lens is

+5.0 D

+(20/3)D

-(10/3)D

+2.0 D

Here, $v = - 60 c m, u = - 12 c m$
$∴$ By using the relation, $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
We have, $\frac{1}{f} = \frac{1}{- 60} - \frac{1}{- 12}$
$\Rightarrow \frac{1}{f} = \frac{1}{15} c m$ or $\frac{100}{15} m$
So, the power of lens, $P = \frac{100}{15} = + \frac{20}{3} D$

Q. The least distance of vision of a longsighted person is 60cm. By using a spectacle lens, this distance is reduced to 12cm. The power of the lens is