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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?
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Solution
The image will be formed at infinity.
Given:
Focal length of the objective, f0 = 30 cm Length of the tube, L = 27 cm
Now,
L = f0 - |fe|
n a Galilean telescope, the eyepiece lens is concave.
Thus, we have:
fe = f0 -L = 30 - 27 = 3 cm
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