Question

the image is formed at the distance of 15cm by a concave lens and focal length 20cm.find the object distance​

Answer 1

The lens formula says that- 1/v -1/u=1/f where, v=image distance=-15 cm,as the image will form behind the lens.            u=object distance=?             f=focal length=-20 cm so after putting these values in the lens formula we wiil be getting u=60cm behind the lens

Answer 2

Answer:

Given f = - 20 cm, (sign convention) , v = - 10 cm

(∴ image formed by concave lens is virtual)

By using lens formula,

or u = - 20

Thus the object is placed at 20 cm from the concave lens