Thermal Radiation
The nature of radiation has puzzled scientists for centuries. Maxwell
proposed that this form of energy travels as a vibratory electric and
magnetic disturbance through space in a direction perpendicular to those
disturbances.
In the diagram, the electric (red) and magnetic (blue) oscillations are
orthogonal to each other - the electric lying in the xy plane; the
magnetic, in the xz plane. The wave is traveling in the x direction. An
electromagnetic wave can be defined in terms of the frequency of its
oscillation, designated by the Greek letter nu (v). The wave moves
in a straight line with with a constant speed (designated as c if it is
moving through a vacuum); the distance between successive 'peaks' of the
wave is the wavelength,
,of the wave and is
equal to its speed divided by its frequency.
The electromagnetic spectrum covers an enormous range in wavelengths,
from very short waves to very long ones.
The only region of the electromagnetic spectrum to which our eye is
sensitive is the "visible" range identified in the diagram by
the rainbow colors.
The sun is not the only object that provides radiant energy; any object
whose temperature is greater than 0 K will emit some radiant energy. The
challenge to scientists was to show how this radiant energy is related to
the temperature of the object.
If an object is placed in a container whose walls are at a uniform
temperature, we expect the object to come into thermal equilibrium with
the walls of the enclosure and the object should emit radiant energy just
like the walls of the container. Such an object absorbs and radiates the
same amount of energy. Now a blackened surface absorbs all radiation
incident upon it and it must radiate in the same manner if it is in
thermal equilibrium. Equilibrium thermal radiation is therefore called black
body radiation.
The first relation between temperature and radiant energy was deduced
by J. Stefan in 1884 and theoretically explained by Boltzmann about the
same time. It states:
where the total energy is per unit area per second emitted by the back
body, T is its absolute (thermodynamic) temperature and 
is the Stefan-Boltzmann constant.
The great question at the turn of the century was to explain the way
this total radiant energy emitted by a black body was spread out into the
various frequencies or wavelengths of the radiation. Maxwell's
"classical" theory of electromagnetic oscillators failed to
explain the observed brightness distribution. It was left to
Max
Planck to solve the dilemma by showing that the energy of the
oscillators must be 
