Question

the image of an object formed by a lens is of magnification is -1. if the distance between the object and its image is 60 cm, what is its focal length of the lens?if the object moved 20 cm toward the lens, where would the image be formed? state reason

Answer 1

when object is kept at 2f then a magnification of -1 is observed 
so image distance = object distance ( magnitude) = 2f
total distance between image and object = 2f + 2f = 4f = 60
hence focal length = 15 cm 

Answer 2

Magnification, m = -1
Distance between object and image, u + v = 60 cm

Magnification (m) is given by
m=vu
⇒−1=vu
⇒v=−u


Now, we have,
− u + v = 60
⇒−u−u=60
⇒−2u=60
⇒u=−30 cm
So, v=−(−30)=30 cm
Using lens formula
1v−1u=1f
⇒1f=130−1−30=230
⇒1f=115
⇒f=15 cm