A Galilean telescope is $18 cm$ long when focused to form an image at infinity. If the objective lens has a focal length of $20 cm$ , what is the focal length of the eyepiece?

5 cm

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1 cm

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2 cm

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3 cm

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Solution

The correct option is C $2 cm$
Given,
$f_{0} = 20 cm, L = 18 cm$

For Galilean telescope, a concave lens is used as eyepiece.

So, $L = f_{0} - | f_{e} |$

$\Rightarrow | f_{e} | = f_{0} - L = 20 - 18 = 2 cm$

Hence, $(C)$ is the correct answer.

Key Concept: In Galilean telescope, concave lens is used as an eyepiece.

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A Galilean telescope is 18 cm long when focused to form an image at infinity. If the objective lens has a focal length of 20 cm, what is the focal length of the eyepiece?

The correct option is C 2 cmGiven, f0=20 cm, L=18 cm For Galilean telescope, a concave lens is used as eyepiece. So, L=f0−|fe| ⇒|fe|=f0−L=20−18=2 cm Hence, (C) is the correct answer. Key Concept: In Galilean telescope, concave lens is used as an eyepiece.

A Galilean telescope is $18 cm$ long when focused to form an image at infinity. If the objective lens has a focal length of $20 cm$ , what is the focal length of the eyepiece?