Two towers on the top of two hills are 40 km apart. The line joining them presses 50 m above a hill half way between the towers. What is the longest wavelength of radiowaves which can be send between the towers without appreciable diffraction effects?

Explanation

Distance between the towers, d=40
Height of the line joining the hills, d=50
Thus, the radial spread of the radio waves should not exceed 50 km
since the hill is located halfway between the towers, Fresnel's distance can be obtained as,
`Z_(p)=20 km =2xx10^(4)m`
Apeture can be given by the relation,
`Z_(p)=(a^(2))/(lambda)`
`:. lambda=(a^(2))/(Z_(p))`
`=((50)^(2))/(2xx10^(4))=1250xx10^(-4)=0.1250m =12.5m`
Therefore, the wavelength of the radio waves is `12.5 m`