Page 509 - Bird R.B. Transport phenomena

§16.1 The Spectrum of Electromagnetic Radiation 489

log 10 A
17
16
15
14
l k m - 13
12
Electrical conductor
ц Radio waves - carrying alternating
current
1 m- 10

1 cm-
1 mm- 7

6
Far infrared Molecular rotations
5 I
Near infrared Molecular vibrations
1/x,- 4 .
i i Visible -л Displacement of outer
3 Ultraviolet — electrons of an atom
i
2
1 m/x- 1 x-rays Displacement of inner
\
,
electrons of an atom
J
1 A- 0
-1 Displacement of nucleons
y-rays . r
-2 in an atomic nucleus
u
- 3 -
Fig. 16.1-1. The spectrum of electromagnetic radiation, showing
roughly the mechanisms by which various wavelengths of radiation
8
are produced (1 A = Angstrom unit = 1СГ cm = 0.1 nm; l/x = l mi-
6
cron = 10" m).

Here h = 6.626 X 10 34 J-s is Planck's constant. From these two equations and the infor-
mation from Fig. 16.1-1, we see that decreasing the wavelength of electromagnetic radia-
tion corresponds to increasing the energy of the corresponding photons. This fact ties in
with the various mechanisms that produce the radiation. For example, relatively small
energies are released when a molecule decreases its speed of rotation, and the associated
radiation is in the infrared. On the other hand, relatively large energies are released
when an atomic nucleus goes from a high energy state to a lower one, and the associated
radiation is either gamma- or x-radiation. The foregoing statements also make it seem
reasonable that the radiant energy emitted from heated objects will tend toward shorter
wavelengths (higher energy photons) as the temperature of the body is raised.
Thus far we have sketched the phenomenon of the emission of radiant energy or pho-
tons when a molecular or atomic system goes from a high to a low energy state. The re-
verse process, known as absorption, occurs when the addition of radiant energy to a
molecular or atomic system causes the system to go from a low to a high energy state.
The latter process is then what occurs when radiant energy impinges on a solid surface
and causes its temperature to rise.