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  • Please,please help me A astronomical telescope has objective and eyepiece of focal lengths 40 cmand 4 cm respectively. To view an object 200 cm away from the objective,the lenses must be separated by a distance :

A astronomical telescope has objective and eyepiece of focal lengths 40 cm and 4 cm respectively. To view an object 200 cm away from the objective, the lenses must be separated by a distance :

  • Option 1)

    67.3 cm

  • Option 2)

    46.0 cm

  • Option 3)

    50.0 cm

  • Option 4)

    54.0 cm

 

Answers (1)

best_answer

As we discussed in

For astronomical telescope

f_{o}=40cm\\f(e)=4cm

Object distance for objective lens

\mu_{o}=-200cm

For 1st image : \mu_{o}=-200,\:\:\:\:\:\:\:f_{o}=40cm

\therefore\:\frac{1}{V_{o}}-\frac{1}{\mu _{o}}\:=\frac{1}{f_{o}}\:\:\:\:\: or \frac{1}{V_{o}}=\frac{1}{f_{o}}+\frac{1}{\mu _{o}}

or \frac{1}{V_{o}}=\frac{1}{40}-\frac{1}{200}=\frac{4}{200}

V_{o}\:=\:50cm.

Since image will form at focal point of eyepiece, hence distance of this image from eyepiece = 4 cm

Hence distance between objective and eyepiece = (50+4) cm

                                                                                = 54 cm


Option 1)

67.3 cm

This option is incorrect.

Option 2)

46.0 cm

This option is incorrect.

Option 3)

50.0 cm

This option is incorrect.

Option 4)

54.0 cm

This option is correct.

Posted by

prateek

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