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

Correct answer: B. +(20 / 3) D Using, (1)/(f)=(1)/(v)-(1)/(u) Here, v=-60 ~cm, u=-12 ~cm . gathered (1)/(f)=(1)/((-60))-(1)/((-12))=(-1)/(60)+(1)/(12)=(1)/(15) (1)/(f)=(1)/(15) f=15 ~cm gathered Power, P=(100)/(15)=(20)/(3) D

AIIMS 2017

Exam: AIIMS
Year: 2017
Subject: Physics
Chapter: Ray Optics

AIIMS 2017 · Physics · Ray Optics

The least distance of vision of a longsighted person is 60 ~cm . By...

The least distance of vision of a longsighted person is

60 \mathrm{~cm}

. By using a spectacle lens, this distance is reduced to

12 \mathrm{~cm}

. The power of the lens is

Options

A $+5.0 \mathrm{D}$
B $+(20 / 3) \mathrm{D}$
C $-(10 / 3) \mathrm{D}$
D $+2.0 \mathrm{D}$

Correct answer

B. $+(20 / 3) \mathrm{D}$

Step-by-step solution

Using,

\frac{1}{f}=\frac{1}{v}-\frac{1}{u}

Here,

v=-60 \mathrm{~cm}, u=-12 \mathrm{~cm}

\begin{gathered}\therefore \quad \frac{1}{f}=\frac{1}{(-60)}-\frac{1}{(-12)}=\frac{-1}{60}+\frac{1}{12}=\frac{1}{15} \\ \frac{1}{f}=\frac{1}{15} \Rightarrow f=15 \mathrm{~cm}\end{gathered}

Power,

P=\frac{100}{15}=\frac{20}{3} \mathrm{D}

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