Question

Objective lens of a telescope has focal length $100\,cm.$ When the final image is formed at least distance of distance between the lenses is $105\,cm.$ Calculate the focal length of eye piece.

• A. $5\,cm$
• B. $6.25\,cm$
• C. $3\,cm$
• D. $2\,cm$

Answer 1

Answer: (B)

$L=f_{0}+\left|u_{e}\right|$
$\left|u_{e}\right|=L-f_{0}=5\,cm$
$\frac{1}{v_{e}}-\frac{1}{u_{e}}=\frac{1}{f_{e}}$
$\frac{1}{- 25}-\frac{1}{- 5}=\frac{1}{f_{e}}$
$f_{e}=6.25\,cm$