Question

In a laboratory four convex lenses $L_1 L_2, L_3$ and $L_4$ of focal lengths $2, 4, 6$ and $8$ cm, respectively are available. Two of these lenses from a telescope of length $10$ cm and magnifying power $4$. The objective and eye lenses are respectively

• A. $L_2, L_3$
• B. $L_1, L_4$
• C. $L_1, L_2$
• D. $L_4, L_1$

Answer 1

Answer: (D)

$L_4, L_1$

Answer 2

Length o f telescope tube $L=f_{0}+f_{e}$ or $10=f_{0}+f_{e}\dots(i)$ Magnifying power $M=\frac{f_{0}}{f_{e}}$ or $4=\frac{f_{0}}{f_{e}}\dots(ii)$ Solving (i) and (ii), we get $f_{0}=8\,cm$, $f_{e}=2\,cm$ From the table, $L_{4}$ is used as a objective whereas $L_{1}$ is used as a eye piece