Question

a concave mirror of focal length 30cm is placed at a distance of 90cm from the wall.how far from the wall should an object be placed so as to get its real image on the wall is

Answer 1

focal length of concave mirror is generally negative .

so,


f = -30 cm


use mirror formula ,

1/v + 1/u = 1/f

image form real in the wall e.g distance of image from mirror is 90cm

v = -90 cm

now,

1/(-90) +1/u = 1/(-30)

1/u = 1/90 -1/30
= -1/45

u = -45 cm

hence, object distance from wall = 90-45 =45 cm .

Answer 2

Answer:

Given:-

v = -90 cm

f = -30 cm

To Calculate:-

u = ???

Formula to be used:-

Using mirror Formula,

$\frac{1}{v}+\frac{1}{u}+\frac{1}{f}$

On putting values , we have

⇒ $\frac{1}{u}=\frac{1}{f}-\frac{1}{v}$

⇒ $\frac{1}{u}=-\frac{1}{30}-\frac{1}{(-90)}$

⇒ $\frac{1}{u}=-\frac{1}{45}$

⇒ $u=-45 \: cm$

Thus, the object is at distance of 45 cm from the mirror (90 cm - 45 cm ) from the wall.