Two towers on the top of two hills are 40 km apart . The line joining then passes 50 m above a hill way between the towers . What is the longest wave length of radio waves which can be sent between the towers with out appreciable diffraction effects ?

Question

$\begin{array}{1 1} 1.25\;cm \\ 12.5 \; cm \\ 1.25 \; m \\ 12.5 \;m\end{array} $