Physics - Geometrical Optics Question with Solution | TestHub

As image formed is real therefore lens must be convex,$v=20 \mathrm{c}\mathrm{m}$. Let${ f}_1$, be focal length for this lens.
$\therefore \frac{1}{f_1}=\frac{1}{v}-\frac{1}{u}=\frac{1}{20}-\frac{1}{u}$
After placing it in contact with another lens, the image is shifted$10 \mathrm{c}\mathrm{m}$towards the combination.
$i.e,. v=\left(20-10\right)\mathrm{c}\mathrm{m}=10\mathrm{ }\mathrm{c}\mathrm{m}$
$\therefore \frac{1}{10}-\frac{1}{u}=\frac{1}{f_1}+\frac{1}{f_2}=\left(\frac{1}{20}-\frac{1}{u}\right)+\frac{1}{f_2}\Rightarrow f_2=20 cm$
$\therefore P=\frac{100}{20}{\mathrm{m}}^{-1}=5\mathrm{D}$