If the tube length of astronomical telescope is $105 c m$ and magnifying power is $20$ for normal setting, then the focal length of the objective is _______cm.
(Hint: Optical length of astronomical telescope is given by $L \geq f_{o} + f_{e}$ )

10

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25

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20

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100

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Solution

The correct option is D $100$
Forthe astronomical telescope,
Magnifying power $\frac{f_{0}}{f_{e}}$
Where $f_{0} =$ focal length of objective
$f_{e} =$ focal length of eyepiece.
$\Rightarrow 20 = \frac{f_{0}}{f_{e}} \Rightarrow f_{0} = 20 f_{e}$
And $l = f_{0} + f_{e} 105 = 21 f_{e} \Rightarrow f_{e} = 5 c m$ and $f_{0} = 20 \times 5 = 100 c m$

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If the tube length of astronomical telescope is 105cm and magnifying power is 20 for normal setting, then the focal length of the objective is _______cm.(Hint: Optical length of astronomical telescope is given by L≥fo+fe)

The correct option is D 100Forthe astronomical telescope,Magnifying power f0feWhere f0= focal length of objectivefe= focal length of eyepiece.⇒20=f0fe⇒f0=20feAnd l=f0+fe105=21fe⇒fe=5cm and f0=20×5=100cm

If the tube length of astronomical telescope is $105 c m$ and magnifying power is $20$ for normal setting, then the focal length of the objective is _______cm.
(Hint: Optical length of astronomical telescope is given by $L \geq f_{o} + f_{e}$ )