You are given 3 lenses l1,l2,l3 each of focal length 20 an object at 40cm in front of l1.the final real image is formed at focus i ofl3,find separation between l1,l2,l3 ?Ans) between l1&l2 its 60 & between l2&l3 it can be any value...........for l3 to form image at i, the object must be at infinity, and for that, image formedby l1 must be at the focus of l2,and also image formed by l1 is 40 cm ahead of l1, i.e., the focus of l2(20 cm)so, distance between l1 and l2 is 40+20=60 cm........