Question

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

Answer 1

1/f=1/v-1/u

f=-30(for concave focal length will be in -ve),v=-20,u=?

1/-30=1/-20-1/u

1/u=-1/30+1/20

1/u=-20+30/600

-1/u=10/600

1/u=-1/60

u=-60