Given, focal length of first lens,
f1=+10‌cm Focal length of second lens,
f2=−10‌cm Focal length of third lens,
f3=+30‌cm Object distance from first lens,
u1=−30‌cm Using lens formula for first lens, we get
=− ⇒=− ⇒=−= v1=15‌cm Thus, the image formed by first lens is 15 cm to the right of first lens.
The object distance for second lens will be
u2=15‌cm−5‌cm=10‌cm Using lens formula for first lens, we get
=− ⇒=− ⇒=+=0 ⇒v2=∞ Thus, the object distance for third lens,
u3 = ∞.
Using lens formula for third lens, we get
=− ⇒=− = v3=30‌cm The final image will form 30 cm from the third lens.
The distance of final image from object will be
30+10+5+30=75‌cm Thus, the distance of final image formed from object O is 75 cm.