Environmental Engineering Reference
In-Depth Information
its wavelength decreases; as the temperature falls the intensity decreases and the
wavelength increases (Figure 2.7). In addition, the amount of radiation reaching any
object is inversely proportional to the square of the distance from the source (Figure 2.8).
This distance decay factor accounts for the difference in solar inputs to the various
planets in our solar system.
To a certain extent radiation is able to penetrate matter, as, for example, x-rays, which
can pass through the human body, but most radiant energy is either absorbed or reflected
by objects in its path. Absorption occurs when the electromagnetic waves penetrate but
do not pass through the object; reflection involves the diversion or deflection of the
waves from the surface of objects without any change of wavelength. The ability of an
object to absorb or reflect radiant energy depends upon a number of factors, including the
detailed physical structure of the material, its colour and surface roughness, the angle of
the incident radiation and the wavelength of the radiant energy.
An object that is able to absorb all the incoming radiation is referred to as a
black
body
. Although it has conceptual value, a perfect black body does not exist in reality. All
objects absorb a proportion of incoming energy and reflect the remainder. The amount of
radiation reflected from a surface is called the
albedo
. The term is most frequently used
for the visible part of the spectrum. It is calculated by dividing the amount reflected by
the total amount arriving at a surface and is normally expressed as a percentage. The
colour of the surface determines the amount reflected. Solar collection panels are matt
black to ensure that the maximum amount of short-wave energy is absorbed and
converted to heat. Differences also occur according to the wavelength of the energy. Thus
snow and sand both absorb
long
-
wave radiation
(5-50 µm) quite efficiently, but they
reflect relatively large proportions of
short
-
wave radiation
(0·4-0·8 µm). Indeed, under
constant conditions, it is possible to define the wavelengths that specific materials
selectively absorb and emit, and this knowledge can be used to characterize or identify
materials through remote sensing. It is frequently used in astronomy to determine the
gases present in stars.
Whereas solid substances usually absorb most wavelengths of radiation, gases tend to
be very selective in their absorption and therefore emission wavelengths (Figure 2.9).
This property is very important to Earth, as it means that the atmosphere absorbs and
emits only in certain wavelengths. At other wavelengths, radiation is able to pass right
through the atmosphere with little modification. The atmosphere is composed of gas
molecules, particles of matter such as dust, water droplets and ice crystals. Light waves
striking these obstacles are scattered in all directions, so that radiant energy is scattered
back to space as well as down to the surface. There is no change of wavelength in this
process, known as
scattering
, simply a change of direction for some of the radiant
energy.
The nature of scattering depends upon the size of particles relative to the wavelength
of the incident radiation. Gas molecules are most effective at scattering light in the blue
wavelength. Since gas molecules compose much of the atmosphere, we see the sky as
blue whether we view it from the ground or from space. When the Sun is setting or rising
the radiant energy passes at a lower angle through
