Question

The length of a Galilean telescope is $33\text{cm}$ when focused to form an image at infinity. If the objective lens has a focal length of $35\text{cm}$, what is the focal length of the eyepiece?

• A. $68\text{cm}$
• B. $4\text{cm}$
• C. $8\text{cm}$
• D. $2\text{cm}$

Answer 1

The correct option is D $2\text{cm}$

Given,
Length of telescope, $L=33\text{cm}$

Focal length of Objective lens, $f_o=35\text{cm}$

In a Galilean telescope the objective is convex and the eyepiece is concave as shown in the figure.




In case of normal adjustment,

Length of the telescope, $L=f_o-|f_e|$

$\Rightarrow 33=35-|f_e|$

$\therefore |f_e|=2\text{cm}$

Hence, option $\left(D\right)$ is correct.