Question

An object is placed at a distance of 20 cm from a concave lens of focal length 20cm. The distance of the virtual image of the object from the lens is?

Answer 1

U=-20
F=-20
1/f=1/u+1/v
1/-20=1/-20+1/v
1/-20+1/20=1/v
1/20=1/v
V=20cm

Answer 2

Given:

Object distance (u) =20cm

Focal length (f) =20cm

Image distance (v) =?

Solution:

Image distance is calculated as,

$1/f=1/u+1/v$

or, 1/20 = 1/20 + 1/v

or, 1/v =1/20 - 1/20

or, 1/v = 0

v = 1/0

v = infinity

Hence, the image will be formed at infinity i.e the distance of the virtual image is infinite.