An object is placed 21 cm in fron of a concave mirror of radius of curvature 20 cm.A glass slab of thicknes 3 cm and refractive index 1.5 is palced close to the mirror in the space between the object and the mirror. Find the position of the final image fromed. The distance of the nearer surface of the slab from the mirror is 10 cm.

Explanation

The situation is shown in ure. Because of the refraction at the two surfaces of the slab, theimage of the objet P is formed at `P_1` shifted towards the mirror by a distance
`t(1-1/mu)=(3cm) (1-1/1.5)=1cm`
Thus the rays faling on the concave mirror are diverging from `P_1` which is at 21 cm -1=20 cm from the mirror. But the radius of curvature is also 20 cm, hence `P_1` is at teh centre. The rays therefore fall normally on the mirror and hence, retrace their path. The final image is formed at P itself.