Question

A convex lens of focal length $20cm$ is placed in contact with a concave lens of focal length $10cm.$ calculate power of combination.

• A.
  • 5 D
• B. +2 D
• C. -5 D
• D. -20 D

Answer 1

The correct option is C -5 D

Focal of convex lens$\left(f\right)=+20cm=20/100=+.20m$
44P=1/f=1/.20=100/20=+5D$
focal length of concave lens$\left(f\right)=-10cm=-10/100=-.1m$
$P=1/-.1=-10D$
power of combination$=5-10=-5D$

hence,

option $\left(C\right)$ is correct answer.