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

To avoid obstruction of spreading radio waves by the hill, the radial spread of the beam over the hills 20 km away should not be more than 50 m.
This implies that Fresnel distance, `Z_F = 20 km`
And size of aperture, `a = 50 m`
`:. Z_F = (a^2)/(lambda)`
`implies lambda = (a^2)/(Z_F) = ((50)^2)/(2 xx 10^4) = 125 xx 10^(-3) m = 125 mm`.