A tank is filled with a liquid to a height of 12 · 5 m. The apparent depth of a needle lying at the bottom of the tank is measured to be 9.0 m.…

\[\begin{aligned} & \mathrm{n}=\frac{\text { speed of light in }}{\text { speed of light in }} \\ & \mathrm{n}=\frac{\text { real depth }}{\text { apparent depth }} \\ & \mathrm{n}=\frac{12.5}{9} \ldots \ldots \ldots \ldots . \\ & \begin{aligned} \mathrm{r} & =\frac{3 \times 10^{8} \times 9}{12.5} \\ & =2.16 \times 10^{8} \mathrm{~m} / \mathrm{s} \end{aligned} \end{aligned} \]