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June 9, 2009
Brief Review of Quantum Mechanics
32
Blackbody Radiation
3.1.1
Blackbody radiation refers to electromagnetic (EM) radiation
emitted from a small hole in a cavity with walls maintained at
a certain temperature. A schematic of the experiment is shown in
Fig. 3.1(a). Atoms comprising the wall are continuously emitting
electromagnetic radiation as well as absorbing radiation emitted
by other atoms of the wall. Thus the cavity is filled with electro-
magnetic radiation. At equilibrium, the amount of energy emitted
is equal to the amount of energy absorbed. And the energy den-
sity of the EM field is constant. A small hole in the cavity allows
some EM radiation to escape from the cavity and be detected dur-
ing the experiment. The resultant monochromatic energy den-
sity of the blackbody radiation as a function of the wavelength
typically obtained during the experiment is shown in Fig. 3.1(b).
It can be seen from the plot that for a certain temperature, the
energy density shows a pronounced maximum at a certain wave-
length known as the Wien’s wavelength. The Wien’s wavelength
decreases as the temperature increases. This explains the change
in color of a radiating object as its temperature changes.
According to classical theory, the radiated intensity (I) of the
emitted radiation is given by
1
(3.1)
I(λ, T) ∝
4
λ
where λ is the wavelength of the emitted radiation.
classically the radiated intensity of electromagnetic radiation is
Classical Theory Hence ch03
Blackbody Radiator Experimental Data
Intensity
Detector
Wavelength
(a) (b)
Figure 3.1. (a) Schematic of the Blackbody radiation experiment and (b)
a plot of the energy density of the blackbody radiation at a certain tem-
perature together with the theoretical prediction from classical theory.