ray optics

A circular disc of diameter d lies horizontally inside a metallic hemispherical bowl of radius 'a'. The disc is just visible to an eye looking over the edge. The bowl is now filled with a liquid of refractive index u. Now the whole of the disc is just visible to the eye in same posn. . Show that

d = 2a ( u^2 - 1) / (u^2 + 1)