In Fig,, if the image of object O has to coincide with itself, then where must the object be placed at a distance from the lens?

Explanation

Let F is the effective focal length of this concave mirror. Then,
`(1)/(F)=(1)/(f_(m))-(2)/(f_(1))`
`f_(m)=oo, (1)/(f_(1))=(mu-1)((1)/(R))`
`(1)/(F)=(mu-1)(2)/(R)rArr F=(-R)/(2(u-1)`
`F=(-30)/(2(1.5-1))=-30cm`
So, object should be placed at `2F=60cm` from the lens to coincide the image with the object.