Black Body Radiation

A black body is an object which absorbs all the radiation falling on it. The best way to imagine a black body is to imagine a hollow ball, coated matt black on the inside with a tiny hole in it. All the incident radiation going through the hole is absorbed and is unlikely to be re-remitted out of the hole. A black body in thermal equilibrium (that is, at a constant temperature) emits electromagnetic radiation called black-body radiation. The radiation emitted has a spectrum that is determined by the temperature alone.

Screen Shot 2013-10-29 at 22.32.24

This link is helpful. It shows the shape of the emitted radiation curves at different temperatures which can be changed by moving the sidebar. 

Doubling the temperature increases the area underneath by a factor of 16, because of the Stefan-Boltzmann Law, which  states that the total energy P radiated per unit surface area of a black body across all wavelengths per unit time is proportional to the fourth power of Kelvin temperature.  The constant is the Stefan-Boltzmann constant.

Screen Shot 2013-10-29 at 22.26.51

Notice how the maximum wavelength shortens as temperature increases. An object at 5000K will tend to glow red, at 6000K it will be much bluer.

The area under the curves is a measure of the energy of the spectrum at a particular temperature. Doubling the temperature multiplies the energy emitted by a factor of 16 ( 2 to the fourth power)





About John Vagabond

I have taught physics and math all over the world
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