Question

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

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

Answer 1

Answer: (D)

According to the question
$P_1=\frac{100}{20}=5D,P_2=\frac{100}{25}=4D$
Effective power $P=P_1+P_2$
$=5+4=9D$