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  4. In normal adjustment, for a refracting telescope, the distance between objective and eye piece is 30 cm. The focal length of the objective, when the angular magnification of the telescope is 2, will be:

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Q. In normal adjustment, for a refracting telescope, the distance between objective and eye piece is $30\,cm$. The focal length of the objective, when the angular magnification of the telescope is 2, will be:

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Solution:

$f _0+ f _{ e }=30 $
$m =\frac{ f _0}{ f _{ e }} $
$ 2=\frac{ f _0}{ f _{ e }} \Rightarrow f _0=2 f _{ e }$
So $f _0+\frac{ f _0}{2}=30$
$f _0=20\, cm$