Question

Q-An object of height 1.2m is placed before a concave mirror of focal length 20cm so that a real image is formed at a distance of 60cm from it. Find the position of an object. What will be the height of the image formed?

Answer 1

h= 120cm
f= - 20cm
v= -60cm (real image)

1/f = 1/v - 1/u
-1/u = 1/f - 1/v
= - 1/20 - (-1/60)
= - 1/20+1/60
= - 4/120
= - 1/30

Therefore since - 1/u = - 1/30,
-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 .