Question

In Ramsden eyepiece, the distance between the eye lens and cross wires is $1.1cm$. What is the focal length of the lenses of the eyepiece?

• A. $1.2cm$
• B. $2.2cm$
• C. $2.4cm$
• D. $1.3cm$

Answer 1

The correct option is A $1.2cm$

Ramsden's eyepiece consists of two plano-convex lenses each of focal length $f$ separated by a distance equal to $\frac{2}{3}f$

Also the cross wire is at a distance of $\frac{f}{4}$ from the objective lens.

So distance $d$ of the cross wire form the eye is

$d=\frac{2}{3}f+\frac{f}{4}=1.1cm$

$⟹f=1.2cm$