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
The phenomenon of diffraction occurs when a wave encounters an obstacle or passes through an opening that is on the same order of magnitude as its wavelength. The amount of diffraction depends on the relative size of the obstacle or opening to the wavelength.
Radio waves have long wavelengths, typically ranging from a few centimeters to several meters. The wavelength of light waves, on the other hand, is much shorter, typically ranging from a few hundred nanometers to a few hundred micrometers. This significant difference in wavelength is the main reason why we observe noticeable diffraction with radio waves but not with light waves around buildings.
In the case of radio waves, the large wavelength allows them to easily diffract and bend around obstacles, such as buildings, trees, or mountains. Even though buildings are generally much larger than the wavelength of radio waves, the relatively long wavelength enables them to "wrap" around the edges of the building, reaching the other side and thus maintaining reception for portable radios.
Light waves, with their much shorter wavelength, do not diffract as significantly when encountering obstacles or passing through small openings like building edges. This is because the wavelength of light is on a much smaller scale compared to typical building sizes. Instead of diffracting around obstacles like radio waves, light waves tend to propagate in straight lines due to what is known as the rectilinear propagation of light.
To observe discernible diffraction of light, we need obstacles or openings that are on the same scale as the wavelength of light. Examples of such situations include observing diffraction patterns through narrow slits or edges of objects with micro- to nano-scale features. In those cases, light waves can exhibit diffraction and interference patterns that provide evidence of their wave-like nature.
In summary, the significant difference in the wavelengths of radio waves and light waves accounts for why we can readily observe diffraction of radio waves around buildings but do not see similar effects with light waves.