Question

at what distance from the concave lens of focal length 20cm,should a 6 CM tall object be placed so that it forms an image at 15 cm from the lens?also determine the size of the image formed?

Answer 1

F=-20cm
OO'=6cm
v=15cm

By Lens Formula,
1/v-1/u=1/f
1/15-1/u=1/-20
1/u=1/20-1/15
1/u=3-4/60
1/u=-1/60
u=-60
the object is placed at 60 cm from the lens
Magnification,
M=II'/OO'=v/u
II'/6=15/-60
II'/6=1/-4
||'=3/2
II'=0.15