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3 lenses L1, ...

Question

3 lenses L1, L2 and L3 each of focal length 30 cm are placed coaxially .an object is held at 60cm from the optical centre of lens L1.the final image is formed at the focus os L3.Calculate the separation between 1) L1 and L2 2) L2 and L3

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Solution

F1=f2=f3=30cm
1/f1= 1/v1-1/u1
1/v1=1/f1 +1/u1
= 1/30-1/60
= 1/60
v1=60cm
now image by L3 is formed at focus.
thus object for L3 is placed at infinity.
and hence image firmed by L2 is at infinity.
thus u2= 30cm
object for L2 is placed. at 30cm left from L2 which is 60cm right from L1 as image of L1 is formed at 60cm right of L1

so X1 = distance between L1 and L2
=v1+ u2=60+30= 90cm
Therefore L2 make s image at infinity , any distance between L2 and L3 will make the object for L3 as infinity.
L2 and L3 = any length

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