Two thin lenses of focal lengths ${\mathrm{f}}_1 \mathrm{and} {\mathrm{f}}_2$ are in contact and coaxial. The power of the combination is

If two thin lenses of focal lengths ${\mathrm{f}}_1$ and ${\mathrm{f}}_2$ are placed in contact coaxially, then equivalent focal length of combination is

$\begin{array}{l}\frac{1}{\mathrm{F}}=\frac{1}{{\mathrm{f}}_1}+\frac{1}{{\mathrm{f}}_2}-\frac{0}{{\mathrm{f}}_1{\mathrm{f}}_2}=\frac{1}{{\mathrm{f}}_1}+\frac{1}{{\mathrm{f}}_2}\\ \mathrm{Power}\mathrm{for}\mathrm{the}\mathrm{combination}\mathrm{is}\\ \mathrm{P}=\frac{1}{\mathrm{F}}=\frac{1}{{\mathrm{f}}_1}+\frac{1}{{\mathrm{f}}_2}=\frac{{\mathrm{f}}_1+{\mathrm{f}}_2}{{\mathrm{f}}_1{\mathrm{f}}_2}\end{array}$