Question

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

Answer 1

Here, v ( img distance ) = 60 cm

focal length = 20 cm

h ( ht of object ) = 1.2 m = 120cm

Mirror = concave mirror

To find ::- u ( object distance) and ht of image formed .

Solution ::-

1/v + 1/u = 1/f ........mirror formula

1/-60 + 1/u = 1/(-20)

1/u = 1/60 - 1/20

1/u = 1-3 /60

1/u = -2/60

1/u = -1/30

u = -30 cm

So, the object is formed at 30 cm from the mirror. So the object is between 2F and F as 2F (40cm) > 30 > F (20cm )

ht.of img / ht. of obj = -v/u

h / 120 = -(-60)/-30

h /120 = -2

h = 120 x (-2)

h = -240 cm = -2.4 m

So the ht. of image formed is 2.4 m

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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 .