There are four convex lenses $\mathrm{L}_{1}, \mathrm{~L}_{2}, \mathrm{~L}_{3}$ and $\mathrm{L}_{4}$ of…
There are four convex lenses $\mathrm{L}_{1}, \mathrm{~L}_{2}, \mathrm{~L}_{3}$ and $\mathrm{L}_{4}$ of focal length $2,4,6$ and $8 \mathrm{~cm}$ respectively. Two of these lenses form a telescope of length $10 \mathrm{~cm}$ and magnifying power 4 . The objective and eye lenses are respectively
L1, L2
L1, L4
L2, L3
L4, L1
Solution
$\begin{array}{l}
\mathrm{f}_{0}+\mathrm{f}_{\mathrm{e}}=10 \\
\frac{\mathrm{f}_{0}}{\mathrm{f}_{\mathrm{e}}}=4 \quad \therefore \mathrm{f}_{0}=4 \mathrm{f}_{\mathrm{e}} \\
5 \mathrm{f}_{\mathrm{e}}=10 \quad \therefore \mathrm{f}_{\mathrm{e}}=2 \quad \mathrm{f}_{0}=8
\end{array}$
So, $\mathrm{L}_{4}$ and $\mathrm{L}_{1}$ were used