Question

An object is placed in front of a concave mirror of focal length 20cm. The image formed is 3 times the size of an object. Calculate two possible distances of the object from the mirror​

Answer 1

$\huge\mathcal\color{teal} Answer :-$

the mirror formula is given as :-

1/f = (1/v) + (1/u)  (1)

here

f = focal length = -20cm

v = image distance 

u = object distance

we will assume that the following is given :-

hi = 3ho or

hi / ho = 3

hi = image height

ho = object height

now as,

hi / ho = -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

Answer 2

$\bf\huge{Question:-}$

An object is placed in front of a concave mirror of focal length 20cm. The image formed is 3 times the size of an object. Calculate two possible distances of the object from the mirror

$\bf\huge{answer:-}$

the mirror formula is given as :-

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

here

f = focal length = -20cm

v = image distance

u = object distance

we will assume that the following is given :-

hi = 3ho or

hi / ho = 3

hi = image height

ho = object height

now as,

hi / ho = -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