Q. In a laboratory four convex lenses $L_{1}\ ,\ L_{2}\ ,\ L_{3}andL_{4}$ of focal length 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) $L_{4}\ ,\ L_{1}$
Explanation
(d) Length of tube = 10 cm
$f_{0} + f_{e} = 10\ cm$
Magnification
$m = \frac{f_{0}}{f_{e}} = 4$
$f_{0} = 4f_{e}$
Putting in Eq. (I)
$5f_{e} = 10\ cm$
$or\ f_{e} = 2\ cm$
$and\ \ \ f_{0} = 8\ cm$
$f_{0} = 8cm,\ f_{e} = 2cm$
Hence, $L_{4}\ $and $L_{1}$ will be used.
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.