Radio Wave Propagation
Diffraction and scattering

The spreading out of a wave around an obstacle is called diffraction . This spreading is sometimes referred to as bending around an obstacle. However, to avoid possible confusion with refraction, which is an entirely different process, the term diffraction will be used here. Radio waves undergo both small-scale and large-scale diffraction. An example of small-scale diffraction is radio waves in a WLAN spreading around indoors. An example of large-scale diffraction is radio waves spreading around a mountain peak, to an inaccessible area.

A different effect occurs when light hits small particles. Depending on the frequency of the light and the size and composition of the particles, a phenomenon called scattering is possible. Scattering typically results in the redirection of the incoming wave energy into directions other than the intended direction.

The sun gives off visible and other EM waves. If there were no atmosphere, the light would come straight from the sun and the rest of the sky would be dark, except for other stars. This is exactly the view from the moon. Yet, on earth the sky is blue. This is because the molecules in the atmosphere scatter blue light, much more than the other colors. The result is that while the sunlight of most colors comes straight in towards an observer on earth, the blue light is scattered over such a large portion of the atmosphere that the atmosphere essentially appears to glow blue. This is illustrated in Figure .

Light scatters off of all kinds of materials. Figure shows why a cloud looks white, which is another effect of scattering.