How much water should be filled in a container 21 cm in height. so that it appears half filled when viewed from thetop of the container? (given that μw  a  =4/3)

To see the container half-filled from top, water should be filled up to height x so that bottom of the container should appear to be raised upto height (21- x).As shown in figure apparent depth h' = (21 - x) Real depth h= x∴μ=hh'⇒43=x21-x⇒12 cm