Question

an object of height 1.2 M is placed before a concave mirror of focal length 20cm so that a real images form at a distance of 60 cm from 8 find the position of an object what will be the height of the image formed

Answer 1

Given,

Height of object, ho = 1.2m =120cm

Focal length, f= -20cm

Image distance, v = -60cm

Object distance, u =?

Height of image, hI =?

Sol.   By mirror formula,

                                          1/f = 1/u + 1/v

                                        -1/20= 1/u – 1/60

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

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

                                        1/u = - (3/60)                                                  ( By, LCM)

                                          U = - 20cm (answer)

         For image height,

                                         hI/ho = - v/u

                                          HI = - (60/20)x 120

                                        HI = - 3x 120

                                       HI = - 360cm = - 3.6m   (answer)

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 .