Radiation Basics
10
The modes of energy transport discussed so far (conduction, convection,
and latent heat) all are somewhat intuitive. Radiative energy transport,
on the other hand, is not intuitive at all. Radiant energy is transferred
by photons, discrete bundles of electromagnetic energy that travel at the
speed of light (c = 3 x 101° mls in vacuum) and behave both as particles
and waves. These photons are emitted or absorbed by matter as a result
of quantum jumps in electronic energy levels in atoms, or changes in
vibrational and rotational energy levels in molecules. The wavelength of
the radiation is uniquely related to the photon energy in an equation due
to Planck:
where h is Planck's constant (6.63 x
J S) and h is the wavelength of
the photon. Thus green photons, having a wavelength of 0.55pm would
have an energy
The energy transferred by a single photon is not generally of interest,
but often the energy content of a mole of photons is. This is obtained by
multiplying the energy per photon by Avagadro's number (6.023 x 1 0 ~ ~ ) .
The energy content of photons at 0.55pm wavelength is
23 photons
J
J
6.023 x 10 --- x 3.6 x lo-'' - = 2.17 x lo5 - .
mol
photon
mol
This kind of calculation allows conversions between amounts of radiant
energy and numbers or moles of photons for a particular wavelength.
The energy of photons could also be expressed as a function of frequency v of the radiation, since vh = c, to give e = h v. Frequency, rather
than wavelength is used in some treatments of environmental radiation
(Gates, 1980). Advantages of using frequency are a more symmetrical
presentation of absorption bands and the ability to show both solar and
thermal radiation on a single graph. These advantages are offset somewhat by the loss of detail in the longwave portion of the spectrum and the
10
The modes of energy transport discussed so far (conduction, convection,
and latent heat) all are somewhat intuitive. Radiative energy transport,
on the other hand, is not intuitive at all. Radiant energy is transferred
by photons, discrete bundles of electromagnetic energy that travel at the
speed of light (c = 3 x 101° mls in vacuum) and behave both as particles
and waves. These photons are emitted or absorbed by matter as a result
of quantum jumps in electronic energy levels in atoms, or changes in
vibrational and rotational energy levels in molecules. The wavelength of
the radiation is uniquely related to the photon energy in an equation due
to Planck:
where h is Planck's constant (6.63 x
J S) and h is the wavelength of
the photon. Thus green photons, having a wavelength of 0.55pm would
have an energy
The energy transferred by a single photon is not generally of interest,
but often the energy content of a mole of photons is. This is obtained by
multiplying the energy per photon by Avagadro's number (6.023 x 1 0 ~ ~ ) .
The energy content of photons at 0.55pm wavelength is
23 photons
J
J
6.023 x 10 --- x 3.6 x lo-'' - = 2.17 x lo5 - .
mol
photon
mol
This kind of calculation allows conversions between amounts of radiant
energy and numbers or moles of photons for a particular wavelength.
The energy of photons could also be expressed as a function of frequency v of the radiation, since vh = c, to give e = h v. Frequency, rather
than wavelength is used in some treatments of environmental radiation
(Gates, 1980). Advantages of using frequency are a more symmetrical
presentation of absorption bands and the ability to show both solar and
thermal radiation on a single graph. These advantages are offset somewhat by the loss of detail in the longwave portion of the spectrum and the