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A galilien telescope is 27 cm long when focused to form an image is at infinity.If the focal length of obdject has a focal length of 30cm ,What is the focal length of eyepiece.

A galilien telescope is 27 cm long when focused to form an image is at infinity.If the focal length of obdject has a focal length of 30cm ,What is the focal length of eyepiece.

Grade:12

2 Answers

Saurabh Koranglekar
askIITians Faculty 10336 Points
2 years ago
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
Vikas TU
14149 Points
2 years ago
 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

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