posted by Anonymous .
Radio waves are able to diffract readily around buildings, as anybody with a portable radio receiver can verify. However, light waves, which are also electromagnetic waves, undergo no discernible diffraction around buildings. Why not?
diffraction is noticable for long wave, not for short waves.
Here are few additional comments to Bob Pursley's correct answer
Waves spread sideways a distance x beyond edges of objects by a characteristic distance
y = sqrt(wavelength*X)
This is called Fresnel diffraction
There is another type of diffraction in the "far field" past objects of size D that block waves. It is called Fraunhofer diffraction and has a characteristic spread angle
theta = wavelength/D