In the medium wave radio band, waves sent out by the transmitter, T, may reach the receiver, R, by two routes: the ground wave travels direct, while the sky wave is returned to earth by the layers of ionized gas collectively called the ionosphere. 1) The height h of the ionosphere may vary over a few seconds. Explain how the presence of the 2 waves can cause the signal at the receiver to rise and fall in intensity as h varies. 2) If d =120 km, and h=80 km at a particular moment, determine the path difference for direct and reflected waves. 3) Hence find, for radio waves of wavelength 1500 m, whether the interference is constructive, destructive or none of the above