Question

The focal length of concave mirror is 12cm where should an object of breadth 4cm be placed so to obtain a real image of breadth 1cm

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