An object is placed on the principle axis of a concave mirror of focal length 10cm at a distance of 21cm from it. A glass slab is placed between the mirror and the object as shown in figure.
Find the distance of final image formed by the mirror.

Explanation

Shift `= 3(1 - (1)/(3//2))`
For mirror object is at a distance `= 21 - 3 (1 - (1)/(3//2)) = 20cm`
Object is at the centre of curvature of mirror. Hence the light rays will retrace and image will be formed on the object itself.