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 form 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)

Length of tube $=10\, cm$
$ f_{o}+f_{0}=10\, cm$
Magnification $m=\frac{f_{0}}{f_{e}}=4$
$f_{o}=4 f_{e}$
Putting in Eq. (i),
$5 f_{e} =10\, cm$
or $f_{e} =2\, cm$
and $f_{o} =8\, cm$
$f_{o} =8\, cm , f_{e}=2\, cm$
Hence, $L_{4}$ and $L_{1}$ will be used.