Question

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

• A. $\left(1/9\right)D$
• B. $9D$
• C. $\left(1/45\right)D$
• D. $45D$

Answer 1

The correct option is B $9D$

Given that,

Focal length $F_1=20cm=0.2m$

Focal length $F_2=25cm=0.25m$

We know that,

The power is for focal length of $0.2m$

$P=\frac{1}{f}$

$P_1=\frac{1}{f_1}$

$P_1=\frac{1}{0.2}$

$P_1=5D$

Now, for focal length $0.25m$

$P_2=\frac{1}{f_2}$

$P_2=\frac{1}{0.25}$

$P_2=4D$

So, the effective power of the combination is

$5D+4D=9D$

Hence, the effective power of the combination is $9D$