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.