Water (with refractive index = $$\frac{4}{3}$$) in a tank is 18 cm deep. Oil of refractive index $$\frac{7}{4}$$ lies on water making a convex surface of radius of curvature ‘R = 6 cm’ as shown. Consider oil to act as a thin lens. An object ‘S’ is placed 24 cm above water surface. The location of its image is at ‘x’ cm above the bottom of the tank. Then ‘x’ is