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Solution


Q.

An 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           [2016]
1

50.0 cm

 

2

54.0 cm 

 

3

37.3 cm

 

4

46.0 cm

 


Ans.

(2)

Here fo=40cm, fe=4cm

Tube length (l) = Distance between lenses = vo+fe

For objective lens,

       uo=-200cm,  vo= ?

       1vo-1uo=1fo  or  1vo-1-200=140

or    1vo=140-1200=4200  ∴  vo=50cm

∴  l=50+4=54cm