Question

a convex lens have focal length 25cm at what distance should object from the lens be placed so that if it forms an image at 20cm distance from the lens? also find the magnification produced by the lens ?

Answer 1

1/f=1/v-1/u
since u is negative for convex lens by sign convention
1/f=1/v+1/u
1/25=1/20+1/u
1/25-1/20=1/u
u=-100
- sign indicates that object is placed on the left side of convex lens that is opposite side of image
magnification=20/-100=-1/5

Answer 2

sorry for handwriting
god help u to understand my hanedwriting.