find the position of an object when placed in front of a concave mirror of focal length 20 cm produces a virtual image,which is twice size of object.
given:
focal length
f= -20cm
and magnification
m = +2 = -v/u
or
v= -2u_______(1)
now, the mirror relation is
1/f = 1/v + 1/u
or
-1/20 = u + v/uv
= u-2u /-2u2 [substituting value of v from eq. (1)]
thus, image distance will be
u= -10cm