Question

an object of hight 1.2m is placed before a concave mirror of focal length 20cm and real image is formed at a distance of 60cm find the position of object

Answer 1

v=60cm
f=20cm
u=?
so,
1/v+1/u=1/f
1/-6-1/-u=1/-20
1/-u=-1/20+1/60
-1/u=-3+1/60
-u=-30
u=30cm

Answer 2

Answer:

u = 30 cm

$\display \text{h _i = 2.4 cm}$

Explanation:

We have given :

Focal length = 20 cm

Image distance =  60 cm

We have to find object distance .

We know :

1 / f = 1 / v + 1 / u

Since image formed is real :

f = - 20 cm and v = - 60 cm

1 / u = 1 - 3 / 60

u = - 30 cm

Hence object distance is 30 cm .

Also given object height = 1.2 cm

We know :

h_i / h_o = - v / u

h_i / 1.2 = - 60 / 20

h_i = - 2.4 cm

Hence image height is 2.4 cm .