Question

where should an object be placed in front of a concave mirror of focal length 20 cm so as to obtain a two times magnified real image

Answer 1

Since it is 2 times magnified, m = -2

so -v/u = -2

similarly, 1/v + 1/u = 1/f

so 1/v + 1/u = 1/-20

-v/v + -v/u = -v/-20 (multiply throughout by -v)

-1 -2 = v/20

-3 = v/20

v = -60

-2 = -v/u

-2 = 60/u

u = -30

so the object must be placed 30 cm from the mirror. 

Answer 2

heya friends.!!

focal length "f"=-20
magnification "m"=-2times.

as you know that

magnification =f/f-u by using mirror formula obtain relation between m and f. -

so using this formula,

-2=-20/-20-u

40+2u=-20.
u=-60/2

u=-30cm


u " distance of object =- -30cm

so if you object be placed at the distance of 30 cm then the two times magnified image will be obtain.

hope it help you.
☺

@rajukumar☺