A point source of light S is placed at the bottom of a vessel containg a liquid of refractive index `5//3.` A person is viewing the source from above the surface. There is an opaque disc of radius `1 cm` floating on the surface. The centre of the disc lies vertically above the source S. The liquid from the vessel is gradually drained out through a tap. What is the maximum height of the liquid for which the source cannot at all be seen from above?