An object AB of height 2 cm is placed at distance 63 cm from a concave mirror of radius of curvature 40 cm. A slab of refractive index 43 and thickness 12 cm is placed between the object and the mirror.

The object shift due to slab is x=t1−1u=121−34=3 cm The image A1B1 formed due to refraction through slab behaves as object for concave mirror.For concave mirror, u=−60cm,f=−20cm   According to mirror formula,∵1v+1u=1f or 1v−160=−120Or  1v=160−120=1−360 ∴v=−30 cm m=A1B2A1B1=−vu=−−30−60 ∴A2B2=−1 cm A2B2behaves as virtual object for second times refraction through slab. The final image  is shifted 3cm leftward from A2B2 due to second times refraction through slab. Thus, final image is real, 1 cm high inverted and 33 cm from the concave mirror. The ray diagram is shown in the figure.