Question

A compound lens is made of two lenses in contact having power +12.5 D and -2.5 D. Find the focal length and power of the combination ?

Answer 1

$$\begin{gathered} \mathrm{power}\mathrm{of}\mathrm{the}\mathrm{combination}\mathrm{of}\mathrm{the}\mathrm{lenses}: \\ \mathrm{P}={\mathrm{P}}_1+{\mathrm{P}}_2 \\ =12.5-2.5 \\ =10\mathrm{D} \\ \mathrm{Focal}\mathrm{length}\mathrm{of}\mathrm{the}\mathrm{combination}: \\ \mathrm{f}=\frac{100}{\mathrm{P}} \\ =\frac{100}{10} \\ =10\mathrm{cm}\left(\mathrm{or}\right)0.1\mathrm{m} \end{gathered}$$