Question

An object of height 1.2cm 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 of an object. What will be the height of the image formed?

Answer 1

Object height (h)=1.2cm
Focal length (f)=-20cm
Image distance (v)=-60cm

By using the mirror formula,
1/v+1/u=1/f
1/-60+1/u=1/-20
1/u=1/-20 -1/-60
1/u=3/-60 - 1/60 (LCM=60)
1/u=2/-60
u=-60/2
u=-30cm

let h' be the height of image formed

Using the formula,
h'/h=-v/u
h'/1.2=60/-30
-30h'= 60×1.2
-30h'=72
h'=72/-30
h'=-2.4cm

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 .