Question

A convex lens and a concave lens, each having same focal length of $25\, cm$, are put in contact to form a combination of lenses. The power in diopters of the combination is

• A. 25
• B. 50
• C. infinite
• D. zero

Answer 1

Answer: (D)

Focal length of combination of lenses placed in contact is
$\frac{1}{F}=\frac{1}{f_{1}}+\frac{1}{f_{2}}$
For convex lens, $f_{1}=25\, cm$
For concave lens, $f_{2}=-25\, cm$
Hence, $\frac{1}{F}=\frac{1}{25}+\frac{1}{-25}=\frac{1}{25}-\frac{1}{25}=0$
$F=\frac{1}{0}=\infty$
Hence, power of combination, $P=\frac{1}{F}=0 \,D$