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
• C. 13.5
• D. 15.5

Answer 1

Answer: (C)

13.5

Answer 2

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