Question

An object is at height1.2m is placed before a concave mirror of focal length20cm so that a real image is formed at a distance of 60cm from it. Find tje position of an object. What will be the height of the image formed?

Answer 1

We know that,

Focal length, f = -20 cm (negative sign because it is concave mirror)

image distance, v = -60 cm (negative sign because it is a concave mirror)

Using mirror formula,

1/f = 1/v + 1/u

1/u = 1/f - 1/v

1/u = 1/-20 + 1/60 (v becomes positive because (-) * (-) = (+)

1/u = -3+1 / 60

1/u = -2 / 60

Therefore u = -60/2 = -30 cm.

Height of image formed,

h'/h = -v/u

h' = -v/u * h

h' = -(-60) / 30 * 1.2

h' = 2 * 1.2

h' = 2.4 cm

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 .