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