Question

In Ramadens eye-piece, the two plano-convex lenses each of focal length/, are separated by a distance 12 cm. The equivalent focal length (in cm) of the eye lens:

• A. 10.5
• B. 12
• C. 13.5
• D. 15.5

Answer 1

Answer: (C)

Ramsdens eye-piece consists of two co-axial plano-convex lenses of focal length $f$ and $f_1$ separated by a distance $\frac{2f}{3},$ $\frac{2f}{3}=12$ $\Rightarrow$ $f=\frac{12\times 3}{2}=18cm$ So, equivalent focal length $\frac{1}{F}=\frac{1}{f}+\frac{1}{f}-\frac{d}{f^2}$ $=\frac{1}{18}+\frac{1}{18}-\frac{12}{{(18)}^2}$ $=\frac{1}{9}-\frac{1}{27}$ $=\frac{2}{27}$ $\therefore$ $f=\frac{27}{2}=13.5cm$