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 quantized, i.e. the energies can not take any value but must change in steps, the size of each step, or quantum, is proportional to the frequency of the oscillator and equal to hv, where h is the Planck constant. With this assumption, Planck derived the brightness distribution of a black body and showed that it is defined by its temperature. Once the temperature of a black body is specified, the Planck law can be used to calculate the intensity of the light emitted by the body as a function of wavelength. Conversely, if the brightness distribution of a radiating body is measured, then, by fitting a Planck curve to it, its temperature can be determined.

The curves illustrated below show that the hotter the body is, the brighter it is at shorter wavelengths. The surface temperature of the sun is 6000 K, and its Planck curve peaks in the visible wavelength range. For bodies cooler than the sun, the peak of the Planck curve shifts to longer wavelengths, until a temperature is reached such that very little radiant energy is emitted in the visible range.