1 ) An object is placed at a distance of f/2 from a convex lens . The image will be ?

1 ) An object is placed at a distance of  f/2  from a convex lens . The image will be : -

Solution : -

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2 ) If tube length of astronomical telescope is 105 cm and magnifying power is 20 for normal setting , calculate the focal length of objective ?

Solution : -

L = 105 cm  ,  m = 20  ,  Fo = ?

So , we have formula : -  M  =  Fo / Fe  ,

So , it will become : -  Fo  =  20 Fe .

Now we use 2^nd formula : - 

L  =  Fo + Fe ,

105  =  Fe + 20 Fe  ,

So , Fe = 5 cm .

So , now we will find Fo by same formula : -

L  =  Fo + Fe  ,

105  =  Fo + 5  ,

Fo = 100 cm .

So , answers are  Fo = 100 cm  and  Fe = 5 cm .

 

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