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

Focal length of combination of lenses placed in contact is

$\begin{array}{l}\frac{1}{\mathrm{F}}=\frac{1}{{\mathrm{f}}_1}+\frac{1}{{\mathrm{f}}_2}\\ \mathrm{For}\mathrm{convex}\mathrm{lens},{\mathrm{f}}_1=25\mathrm{cm}\\ \mathrm{For}\mathrm{concave}\mathrm{lens},{\mathrm{f}}_2=-25\mathrm{cm}\\ \mathrm{Hence},\frac{1}{\mathrm{F}}=\frac{1}{25}+\frac{1}{-25}=\frac{1}{25}-\frac{1}{25}=0\\ \mathrm{F}=\frac{1}{0}=\mathrm{\infty }\\ \mathrm{Hence},\mathrm{power}\mathrm{of}\mathrm{combination},\mathrm{P}=\frac{1}{\mathrm{F}}=0\mathrm{D}\end{array}$