Question

an object is placed in front of a concave mirror of focal length 0.15m produce a virtual image ,which is twise the size of the object .the position of the object with respect to the mirror is
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Answer 1

Focal lenght of mirror (f)=-0.15m

Magnification(m)=2

m = -(v/u)

2 = -v/u

-2u = v                                                  {eq.1}

Now we will use mirror formulae

1/f=1/u+1/v

-1/0.15=1/u+1/-2u                                     {from eq. no. 1}

-1/15=(2-1) /2u

-1/0.15=1/2u

-2u=0.15

u=-0.15/2

u=-0.075 m=7.5cm

So, the position of object with respect to mirror will be 7.5 cm back of the mirror.