Question

How far should an object be from the pole of a concave mirror of focal length 20cm to fotm a real image whose size is 1/5 th of the size of the object??

Answer 1

Given mirror is concave mirror
thus
f = -20cm
let the size of object= x
thus size of image = x/5

hence magnification = 1/5
now let the object was placed at
-y then image formed will be at
y/5
now using mirror equation
1/v+1/u = 1/20
5/y-1/y = 1/20
4/y = 1/20
y = 80 cm
hence object will be placed at 80 cm from the object .

Answer 2

For concave mirror , f = - 20 cm

Since the image is 1/5th of the object so the magnification m = 1/5

Let the distance of the object be x

So the distance of image is x/5

1/v + 1/u = 1/f
5/x + (-1/x)= 1/20
4/x = 1/20
x = 80 cm

Hence your required answer is 80 cm

Hope This Helps You!