For diffraction of radiowaves not to occur the distance of middle hill should be less than fresnel distance for a slit width ‘a’ of 50 m. $D_F$ = $\frac{a^{2}}{\lambda}$ Distance between one of the towers and the hill halfway in between = $\frac{40}{2}$ km = 20000 m = 20 × $10^{3}$ m Longest wavelength of radiowave which can be sent without appreciable diffraction effect λ = $\frac{a^{2}}{D_F}$ = $\frac{50 \times 50}{20 \times 10^{3}}$ = 12.5 cm Thus wavelength of radio waves longer than 12.5 cm will bend due to the hill in the middle of towers.