A mark is made on the bottom of a vessel, over this mark a glass slab of thickness 3.5 cm and refractive index 7/4 is placed. Now water (refractive index 4/3) is poured into the vessel so that the surface of water is 8 cm above the upper surface of the slab. Looking down normally through the water, the apparent depth of the mark below the surface of water will be

Refractive index = real depth / apparent depth.

Apparent depth = {8 / (4/3)} + {3.5 / (7/4)} = 6 + 2 = 8 cm