# 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

Shashwat Tiwari
20 Points
12 years ago

thats the board exam question..............

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 formed

by 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........

sharat sh
18 Points
11 years ago

WHY CAN''T IT BE 20Cm

Priya aggarwal
19 Points
7 years ago
For lens L1 object is at 40cm and focal length is 20 cm. By using lens formula image is formed at 40 cm. Now image by L1 will act as object for L2As final image is at focus of L3 ,object for L3 would be at infinity. The object o L3 os the image formed by L2. This tells us that object of L2 will be at focus of L2 then only image formed is at infinity So L2 and L3 can be seperated by any valueNow image of L1 is at 40 cm away from it and this image acting as object for L2 is 20 cm away (at focus) of L2 => L1 + L2 = 40 + 20 = 60cm