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

Given:

Focal length of the objective, fo = 30 cm

Length of the tube, L = 27 cm

Now,

L = fo-fe

In a Galilean telescope, the eyepiece lens is concave.

Thus, we have:

fe = fo – L = 30 – 27 = 3 cm

Answer 2

Solution :-

. For the given Galilean telescope,

(When the image is formed at infinity)

F base 0 = 30 cm,

L = 27 cm

Since L = f
base 0 – |f base e|
[Since, concave eyepiece lens is used in Galilean Telescope]
⇒ f base e

= f base 0 – L

= 30 – 27

= 3 cm