Question

Two lenses of power $15D$ and $-5D$ are in contact. The focal length of combination is

• A. $18cm$
• B. $10cm$
• C. $20cm$
• D. $6cm$

Answer 1

The correct option is B $10cm$

We know that,
Combined power:
$P=P_1+P_2=15-5=10D$
Also,
Focal length = $\frac{1}{\text{Power of lens}}$

$\Rightarrow f=1/P=0.1m=10cm$