Question

How for should an object the from the pots of a concave mirror of focal length 20cm to from a real image when size is 1/4 of the size of the object

Answer 1

focal length is taken negative in case of concave mirror

Answer 2

Answer:

   u = -100cm

Explanation:

focal length (f) = -20cm

Magnification (M) = 1 / 4

image distance (v) = v

object distance (u) = u

M = -v / u

Here magnification is negative because it forms a real image

This is because virtual image formed by concave mirror is enlarged whereas according to the question the image is diminished and hence it forms a real image

-(1 / 4) = -v / u

1 / 4 = v / u

u = 4v -------{i}

1 / v + 1 / u = 1 / f  

1 / v + 1 / 4v = - 1 / 20

4 / 4v + 1 / 4v = -1 / 20

5 / 4v = -1 / 20  

4v = -100

v = -25cm

u = 4v = -100cm

∴ The object should be placed 100 cm to the left of the mirror.