Question

Two thin lenses of focal lengths $20cm$ and $25cm$ are placed in contact. The effective power of the combination is

• A. $\frac{1}{9}$$D$
• B. $45D$
• C. $6D$
• D. $9D$

Answer 1

The correct option is D $9D$

$f_1=20cm=0.2m;f_2=25cm=0.25m$
as power of lens is given by,
$P=\frac{1}{f}$
$P_1=\frac{1}{f_1}=\frac{1}{0.2}=5D$,
$P_2=\frac{1}{f_2}=\frac{1}{0.25}=4D$
so,The effective power of the combination is $5D+4D=9D$.
hence,option D is correct.