How much water should be filled in a container 21 cm in height, so that it appears half filled when viewed from the top of the container (given that $\left.{ }_a{\mu }_{\omega }=4/3\right)$

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). $\mu =\frac{h}{h^{\mathrm{\prime }}}\Rightarrow \frac{4}{3}=\frac{x}{21-x}\Rightarrow x=12\mathrm{cm}$