Two towers on top of the two hills are 40 km part. The line joining them passes 50 m above the hill half way between the towers. What is the longest wavelength of radio waves, which can be sent between the towers without appreciable diffraction effects?

Explanation

Maximum wave length,
`lambda =(D)/(a)=(50)/(40xx10^3)`
`=1.25xx10^(-3) m = 12.5 cm`