Question

How far should an object be from the pole of a concave mirror of focal length 20 cm to form a real image whose size isc1/5th of the object.

Answer 1

Answer:120

Explanation:

given,

Focal length(f)= -20

Magnification(m)= -1/5

Object distance(u)= ?

We know

m=-v/u

-1/5=-v/u

-u=-5v

u=5v. ...........i

Now

1/f=1/v+1/u

-1/20=1/v+1/5v

-1/20=5+1/5v

-1/20=6/5v

-5v=120

-v=120/5

Therefore v=-24

Now putting the value of v in .....i,we get. u=5*-24=-120 cm

Thus the object should be placed at 120 cm in front of the mirror

Answer 2

Explanation:

Hence, 120 cm should an object be far from the pole of a concave mirror. Negative sign indicates real and inverted image.