Background

Black Body Radiation

Before we discuss the term "black body radiation", let's first define what's a "black body". A "black body" is a theoretical perfect absorber, which absorbs radiation of all wavelengths falling on it. It reflects no light at normal temperatures and thus appears black. However, like ideal gas in kinetic theory, it is a theoretical model and we may find in reality only "Almost perfect black bodies".

It follows from Prévost's theory of exchanges of 1792 that the best radiation absorber - the black body, is also the best radiation emitter. The radiation emitted by a black body is called, you guess?... Black body radiation. (Straight forward isn't it?) It is also known as "full radiation" or "temperature radiation".

The intensities of the various wavelengths of radiation emitted by a black body depend only on its temperature. We may study the black body radiation spectrum with a suitable spectrometer. A thermopile or a bolometer can be used, as the black body radiation emitted usually consists of infra red, light and ultra violet which all produce a heating effect.

A black body radiation spectrum is shown below:

We may observe that the higher the temperature of a black body, the more energy is emitted in each band of wavelengths. The black body becomes "brighter". Moreover, the radiation emitted at the highest intensity, represented by the peak of the spectrum, doesn't fall in the visible region unless the temperature is very high, over 3700K.

,

where is the wavelength at which the energy radiated is is a maximum and T is the temperature in Kelvin. This is stated in Wien's displacement law.

Therefore as implied by the formula, a hotter black body emits typical radiation with shorter wavelengths. This explains why black bodies at higher temperatures are blue, and those at lower temperatures are red.

"Black holes aren't black - After Hawking they shine!"
Presented by Angie, Matthias and Thorsten
Team C007571,ThinkQuest Internet Challenge 2000.
Last modified: 2000-08-02.