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In a pvc pipe two concave lens of same focal length are joined together. A boy observes moon through this pipe. How will the image of the moon look to the boy..

In a pvc pipe two concave lens of same focal length are joined together. A boy observes moon through this pipe. How will the image of the moon look to the boy..

Grade:11

2 Answers

Amrit Raj
46 Points
7 years ago
Here the effective focal length of the lens can be calculated using the formula,
f_{eq}=f_{1}f_{2}/(f_{1}+f_{2}-d)
In a familiar way,
1/f_{eq}=1/f_{1}+1/f_{2}-d/f_{1}f_{2}
so we can conclude that,f_{eq} is negative .Since f1 and f2 are negative and d is  +ve.
Here f1=f2=-f (given)
thus feq =-f/2
since d=0.
Now using lens formula
1/f_{eq}=1/v-1/u
we can calculate v=-f/2,   put u=infinity in the eqn
So we can conclude the boy sees a virtual erect and a dimished image of the moon.
 
 
Vikas TU
14149 Points
6 years ago
 We can calculate the effective focal length can be calculated using this formulae =
Feff  = f1f2(f1+f2 –d)
1/ Feff = 1/ f1 + 1/ f2 – d/ f1f2
Here f1 and f2 is negative and d is positive.
f1 = f2 = -ve = -f
Since d = 0
Then effective focal length will be = -f/2
Then using the lens formulae
  • 1/f =1/v-1/u
Then from this  v =-f/2
Hence the image is virtual, erect and diminshed

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