Question

A Galilean telescope is 27 cm long when focussed to form an image at infinity. If the objective has a focal length of 30 cm, what is the focal length of the eyepiece?

Answer 1

The image will be formed at infinity.
Given:
Focal length of the objective, f_o = 30 cm
Length of the tube, L = 27 cm
Now,
L = $f_o-\left|f_e\right|$
In a Galilean telescope, the eyepiece lens is concave.
Thus, we have:
f_e = f_o – L = 30 – 27 = 3 cm