Question

the focal length of a concave lens is 30 cm at what distance should we should the object be placed from the lens so that it forms an image at 20 cm from the​

Answer 1

By lens formula,

1/f=1/v-1/u

therefore,

1/u=1/v-1/f

=1/-20-1/-30

(-20 is because a concave lens forms a virtual errect image on the same side of the lens.)

=-30+20/30×20

=-10/600

u=-60cm