Question

In Ramsden eyepiece, the two planoconvex lenses each of focal length $f$ are separated by a distance $12\, cm$. The equivalent focal length (in $cm$ ) of the eyepiece is

• A. 10.5
• B. 12.0
• C. 13.5
• D. 15.5

Answer 1

Answer: (C)

$d=\frac{2 f}{3}$
or $f=\frac{3 d}{2}=\frac{3 \times 12}{2}=18\, cm$
Equivalent focal length is
$f'=\frac{f_{1} f_{2}}{f_{1}+f_{2}}+\frac{f}{4}$
$=\frac{18 \times 18}{18+18}+\frac{18}{4}$
$=9+4.5$
$=13.5\, cm$