Diffraction of Wave
Diffraction is the name given to the mechanism by which waves enter into the shadow of an obstacle.
Diffraction occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave.
A radio wave that meets an obstacle has a natural tendency to bend around the obstacle. The bending, called diffraction, results in a change of direction of part of the wave energy from the normal line-of-sight path. This change makes it possible to receive energy around the edges of an obstacle.