It is worth noting again that every body of condensed matter radiates according to
Eq. (9.38), regardless of its temperature. An object need not be so hot that it glows
conspicuously in the visible region in order to be radiating. The radiation from an
object at room temperature, for instance, is chiefly in the infrared part of the spectrum
to which the eye is not sensitive. Thus the interior of a greenhouse is warmer than the
outside air because sunlight can enter through its windows but the infrared radiation
given off by the interior cannot escape through them (Fig. 9.8).Wien’s Displacement LawAn interesting feature of the blackbody spectrum at a given temperature is the wave-length (^) maxfor which the energy density is the greatest. To find (^) maxwe first express
Eq. (9.38) in terms of wavelength and solve du( )d 0 for (^) max. We obtain in
this way
4.965
which is more conveniently expressed as
(^) maxT 2.898 10 ^3 mK (9.40)
Equation (9.40) is known as Wien’s displacement law.It quantitatively expresses
the empirical fact that the peak in the blackbody spectrum shifts to progressively
shorter wavelengths (higher frequencies) as the temperature is increased, as in
Fig. 2.6.
hc
4.965k
Wien’s
displacement law
hc
kT (^) max
316 Chapter Nine
Figure 9.8The greenhouse effect is important in heating the earth's atmosphere. Much of the short-
wavelength visible light from the sun that reaches the earth's surface is reradiated as long-wavelength
infrared light that is readily absorbed by CO 2 and H 2 O in the atmosphere. This means that the
atmosphere heated mainly from below by the earth rather than from above by the sun. The total
energy that the earth and its atmosphere radiate into space on the average equals the total energy
that they receive from the sun.
Transmitted through
atmosphere SPACE
Given off by
atmosphere
Absorbed by
atmosphere
Given off by
land and sea
ATMOSPHERE
EARTH
Absorbed by
Absorbed atmosphere
by land
and sea
Reflected by
atmosphere
and surface
30
%
51%
19%
45%
51%
64%
Incoming
solar
radiation
100% 6%
bei48482_Ch09.qxd 1/22/02 8:45 PM Page 316