Question

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

• A. 4.5 D
• B. 18 D
• C. 45 D
• D. 9 D

Answer 1

The correct option is D 9 D

First converting fin metres so lens $1$ has

$f=0.2m$ and lens $2$ has $f=0.25m$

Power of lens$1=\frac{1}{0.2}=5D$

Power of lens$2=\frac{1}{0.25}=4D$

Effective power $=P_1+P_2=5D+4D=9D$