Question

the focal length of a concave 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 1

Using lens formula,

1/f = 1/v - 1/u

1/u = 1/v - 1/f

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

1/u = -1/60

u = -60 cm

∴ Object should be placed at a distance of 60 cm from the lens