AM waves have a longer wavelength that allows t hem to bend around obstacles.
Work Step by Step
As you can verify by looking on your radio dial, AM radio waves have a much smaller frequency and a much longer wavelength than FM radio waves. The amount by which waves bend (“diffract”) around an obstacle depends on size of the obstacle compared to the wavelength. Because AM radio wavelengths are large compared to low hills and buildings, AM radio waves readily bend around such obstacles. The same is not true of FM radio waves.