Black Body Radiation
University of Virginia,
Do not all fix’d Bodies, when heated beyond a certain degree, emit Light and shine;
and is not this Emission perform’d by the vibrating motion of its parts?
, published 1704.
Heated Bodies Radiate
We shall now turn to another puzzle confronting physicists at the turn of the century (1900): just
how do heated bodies radiate? There was a general understanding of the mechanism involved—
heat was known to cause the molecules and atoms of a solid to vibrate, and the molecules and
atoms were themselves complicated patterns of electrical charges. (As usual, Newton was on the
From the experiments of Hertz and others, Maxwell’s predictions that oscillating
charges emitted electromagnetic radiation had been confirmed, at least for simple antennas. It
was known from Maxwell’s equations that this radiation traveled at the speed of light and from
this it was realized that light itself, and the closely related infrared heat radiation, were actually
electromagnetic waves. The picture, then, was that when a body was heated, the consequent
vibrations on a molecular and atomic scale inevitably induced charge oscillations. Assuming
then that Maxwell’s theory of electromagnetic radiation, which worked so well in the
macroscopic world, was also valid at the molecular level, these oscillating charges would radiate,
presumably giving off the heat and light observed.
How is Radiation
What is meant by the phrase “black body” radiation? The point is that the radiation from a
heated body depends to some extent on the body being heated. To see this most easily, let’s back
up momentarily and consider how different materials
radiation. Some, like glass, seem to
absorb light hardly at all—the light goes right through. For a shiny metallic surface, the light
isn’t absorbed either, it gets reflected. For a black material like soot, light and heat are almost
completely absorbed, and the material gets warm. How can we understand these different
behaviors in terms of light as an electromagnetic wave interacting with charges in the material,
causing these charges to oscillate and absorb energy from the radiation? In the case of glass,
evidently this doesn’t happen, at least not much. Why not?
A full understanding of why needs
quantum mechanics, but the general idea is as follows: there are charges—electrons—in glass
that are able to oscillate in response to an applied external oscillating electric field,
charges are tightly bound to atoms, and can only oscillate at certain frequencies. (For quantum
experts, these charge oscillations take place as an electron moves from one orbit to another.
course, that was not understood in the 1890’s, the time of the first precision work on black body
It happens that for ordinary glass