CBSE Class 10 Answered

The mirror formula is given by,

1/f = (1/v) + (1/u) .................................(1)

Given that,

f = focal length = -20cm

v = image distance

u = object distance

we will assume that the following is given

h_i = 3h_o

or

h_i / h_o = 3

h_i = image height

h_o = object height

now as

Magnification = h_i / h_o = -v/u = 3

we get

v = -3u

so, using this in eq. (1), we get

-1/20 = -1/3u + 1/u

or

-1/20 = 2 / 3u

or

u = -13.33 cm

thus, the object distance / position of the object will be

u = -13.33 cm