Question

The focal length of a concave mirror is 12 cm.where an object of length 4 cm be placed so that a real image of 1 cm length is formed?

Answer 1

The relationship between focal length and magnification is :

m = f /(f - d₀)

d₀ = object distance.

Magnification = image height / object height.

Magnification = 1/4 = 0.25

0.25 = 12/(12 - d₀)

0.25(12 - d₀) = 12

3 - 0.25d₀ = 12

-0.25d₀ = 9

d₀ = - 9/0.25

d₀ = 36

The object distance = 36 cm

Answer 2

$m = \frac{f}{f - u}$

u=X

m=1/4

1/4=-12/-12-X

=-12-x=48

X=48+12

x=60