Two lens of focal lengths -20 ~cm and +10 ~cm are put in combination, find the power of the combination.

Correct answer: C. +5 D Here f_1=-20 ~cm, f_2=+10 ~cm Focal length of combination is, aligned & (1)/(F)=(1)/(f_1)+(1)/(f_2)=(1)/(-20)+(1)/(10) & (1)/(F)=(1)/(20) =20 ~cm aligned Power of the combination, P= (100)/(F)=(100)/(20)=+ 5 D

AIIMS 2011

Exam: AIIMS
Year: 2011
Subject: Physics
Chapter: Ray Optics

AIIMS 2011 · Physics · Ray Optics

Two lens of focal lengths -20 ~cm and +10 ~cm are put in...

Two lens of focal lengths

-20 \mathrm{~cm}

and

+10 \mathrm{~cm}

are put in combination, find the power of the combination.

Options

A $-1 \mathrm{D}$
B $-2 \mathrm{D}$
C $+5 \mathrm{D}$
D $+2 \mathrm{D}$

Correct answer

C. $+5 \mathrm{D}$

Step-by-step solution

Here

f_1=-20 \mathrm{~cm}, f_2=+10 \mathrm{~cm}

Focal length of combination is,

\begin{aligned}& \frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}=\frac{1}{-20}+\frac{1}{10}\\ & \frac{1}{F}=\frac{1}{20} \RightarrowF=20 \mathrm{~cm} \end{aligned}

Power of the combination, P=

\frac{100}{F}=\frac{100}{20}=+

\mathrm{D}

Practice Ray Optics on Quantrex Academy →

More from Ray Optics