Question

A convex lens of focal length $40cm$ is in contact with a concave lens of focal length $25cm$. The power of the combination, is

• A. + 6.67 D
• B. - 6.5 D
• C. - 1.5 D
• D. + 6.5 D.

Answer 1

Answer: (C)

Given: Focal length of a convex lens $\left(f_1\right)=40cm=0.4m$
and focal length of a concave lens $\left(f_2\right)=-25cm=-0.25m$
(minus sign due to concave lens).
We know that relation for the focal length of
the combination of lenscs
$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}$
$=\frac{1}{0\cdot 4}+\frac{1}{-0\cdot 25}$
$=2.5-4=-1.5D$
or power $(P)=\frac{1}{f}=-1.5D$