Which of the following effects may allow radio signals to travel beyond obstructions between the transmitting and receiving stations?
Why When a radio wave passes over a sharp obstruction like a ridgeline, rooftop or mountain peak, the wavefront bends slightly around the edge and re-radiates into the shadow zone behind it. This is called knife-edge diffraction, and it can put a usable VHF/UHF signal into a valley or behind a hill where there is no direct line of sight. The effect is strongest with a sharp, well-defined edge, and it is one reason a path can work even when the geometry says the receiver is blocked.
Watch out Faraday rotation is the twisting of a signal's polarization as it passes through the ionosphere, which matters on satellite paths but does not get signals around obstacles; Doppler shift is just a frequency change caused by relative motion.
Sharp ridge = sharp edge: the wave bends over the knife edge into the shadow.