For an opening/object of a given size, diffraction is more pronounced for longer wavelengths.
Work Step by Step
This is discussed on page 548, and illustrated in Figure 29.11. The wavelengths of AM/FM radio waves are thousands of times longer than those of visible light waves. AM radio waves (wavelengths of 100+ meters) diffract easily around buildings. FM radio waves diffract with difficulty (their higher-than-AM-radio-frequency means their wavelengths are shorter). Light waves, with wavelengths of roughly 0.00004-0.00007 cm, hardly diffract around buildings at all.