Question

A concave mirror produces a magnification of 1/2 when an object is at 60 cm from it .Where should the object be placed so that a virtual image of double the size is formed by mirror ?

Answer 1

Initial obj. dist = -60(real object)
". magnification (m) = -1/2 (as both obj and image are real)
We know , m= -f/(u-f) [f=focal length]
f= -20cm
Now , obj dist = u'
magnification(m')=2 (as image is virtual of real object)
f=-20 cm
We know , m= -f/(u'-f)
2=20/(u'+20)
u'=-10cm
Hence , the obj should at a distance of 10 cm in front of the mirror.