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Black Body

Any heated body emits electromagnetic waves. The higher is body temperature, the smaller waves it emits. Body in thermodynamic balance with its radiation is called absolutely black. Radiation of absolutely black body depends on its temperature only.

Absolutely black body may not loose its thermal energy. It is capable to completely absorb all radiation falling on it. In 1900, Max Plank derived a formula for calculation of intensity of absolute black body radiation under established temperature (it is shown in bottom right corner of the model).

Austrian physicians Stefan and Bolzmann established a law expressing quantitative ratio between total radiating capacity and temperature of black body:


This law bears the name of Stefan-Bolzmann law. The constant σ = 5,67*10-8 W/(m2*K4) was called Stefan-Bolzmann constant.

All Planck curves have clearly expressed maximum accounting for wavelength.


This law was named Wien Law. Thus, T0 = 5 800 K for the Sun, with maximum at wavelength λmax≈500 nm. This corresponds to yellow light in optic spectrum.

Maximum radiation of absolutely black body is shifting to short-wave part of spectrum upon temperature growth. Hotter star radiates major part of energy in ultraviolet spectrum. Less hot star does the same in infrared spectrum.

The model displays the spectrum of absolutely black body. Main box shows the diagram of radiation spectral intensity dependence on wavelength or radiation frequency. Dependence On group serves for toggling between these two regimes.

Absolutely black body temperature may be set in input box. In this case the diagram will rise or lower and change its color (this color always corresponds to wavelength with maximal radiation). Sun's spectrum (violet line) is given for comparison.

© OpenTeach Software, 2007