Geoscience Reference
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infrared emissions from the Earth itself. Here, the radia-
tion spectra of both the sun and the Earth are described.
Radiation is the emission or propagation of energy
in the form of a photon or an electromagnetic wave.
Whether radiation is considered a photon or a wave
is still debated. A photon is a particle or quantum
of energy that has no mass, no electric charge, and
an indefinite lifetime. An electromagnetic wave is
adisturbance traveling through a medium, such as
air or space, that transfers energy from one object to
another without permanently displacing the medium it-
self.
Because radiative energy can be transferred even in a
vacuum, it is not necessary for gas molecules to be
present for radiative energy transfer to occur. Thus,
such transfer can occur through space, where few gas
molecules exist, or through the Earth's atmosphere,
where many molecules exist.
Radiation is emitted by all bodies in the universe
that have a temperature above absolute zero (0 K). Dur-
ing emission, a body releases electromagnetic energy
at different wavelengths, where a wavelength is the
difference in distance between two adjacent peaks (or
troughs) in a wave. The intensity of emission from a
body varies with wavelength, temperature, and effi-
ciency of emission. Bodies that emit radiation with per-
fect efficiency are called blackbodies. A blackbody is
a body that absorbs all radiation incident upon it. No
incident radiation is reflected by a blackbody. No bodies
are true blackbodies, although the Earth and the sun are
close, as are black carbon, platinum black, and black
gold. The term blackbody was coined because good
absorbers of visible radiation generally appear black.
However, good absorbers of infrared radiation are not
necessarily black. For example, one such absorber is
white oil-based paint.
Bodies that absorb radiation incident upon them
with perfect efficiency also emit radiation with per-
fect efficiency. The wavelength of peak intensity of
emission of a blackbody is inversely proportional to
the absolute temperature of the body. This law, called
Wien's displacement law ,was derived in 1893 by
German physicist Wilhelm Wien (1864-1928). Wien's
law states
10 -6
10 -4
10 -2
10 0
10 2
10 4
10 6
10 8
10 10
10 12
10 36
10 28
10 20
10 12
10 4
10 -4
10 -12
10 -20
10 -28
10 -6
10 -4
10 -2
10 0
10 2
10 4
10 6
10 8
10 10
10 12
Wavelength (
µ
m)
Figure 2.3. Blackbody radiation emission versus
wavelength at four temperatures. Units are watts
(joules of energy per second) per square meter of
area per micrometer wavelength. The 15 million K
spectrum represents emission from the sun's center
(most of which does not penetrate to the sun's
exterior). The 6,000 K spectrum represents emission
from the sun's surface (photosphere) received at the
top of the Earth's atmosphere (not at its surface). The
300 K spectrum represents emission from the Earth's
surface. The 1 K spectrum is almost the coldest
temperature possible (0 K).
Example 2.1
Calculate the peak wavelength of blackbody
radiative emission for both the sun and the Earth.
Solution
The effective temperature of the sun's photo-
sphere is 5,785 K. Thus, from Equation 2.1, the
peak wavelength of the sun's emissions is about
0.5
m. The average surface temperature of the
Earth is 288 K, giving the Earth a peak emission
wavelength of about 10
m.
At any wavelength, the intensity of radiative emis-
sion from an object increases with increasing tempera-
ture. Thus, hotter bodies (e.g., the sun) emit radiation
more intensely than do colder bodies (e.g., the Earth).
Figure 2.3 shows radiation intensity versus wavelength
for blackbodies at four temperatures. At 15 million
K, a temperature at which nuclear fusion reactions
occur in the sun's center, gamma radiation wave-
lengths (10 8
2,897
T (K)
p (
m)
(2.1)
to 10 4
m) and X radiation wave-
lengths (10 4
where
m) of
peak blackbody emission, and T is the absolute temper-
ature (K) of the body. In 1911, Wien received a Nobel
Prize for his discovery.
p is the wavelength (in micrometers,
m) are the wavelengths emitted
with greatest intensity. At 6,000 K, visible wavelengths
(0.38 to0.75
to 0.01
m) are the most intensely emitted wave-
lengths, although shorter ultraviolet (UV) wavelengths
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