Thu January 17, 2013 By: Vibhuti Vibhu

you are given three lenses L1, L2,L3 each of focal length 20cm.An object is kept at 40cm in front of L1 . The final image is formed at the focas \'I\' of L3. find he separation between L1,L2,L3`

Expert Reply
Thu January 17, 2013
V1 =40cm
Here , image by L3 is formed at the focus. So the object should lie at infinity for L3. so  L2 will form image at infinity.So we can conclude that the object  for L2  should be at its focus.
But we have seen above that the image  by L1 is formed at 40cm right of L1 which is at 20 cm left of L2.
so, X1 = distance between L1 + L2 =20 + 40=60cm
again the distance between L2 and L3 doest not matter as the image formed by L2 is at infinity so that X2 can take any value.
Tue February 07, 2017

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