Modern Physics

Black body radiation

Spectrum of a hot body as a function of temperature

The study of thermal radiation began when the German physicist Robert Kirchhoff, when analyzing the relationships between absorbed heat and emitted heat, proposed two fundamental laws for the study of thermal radiation.

The first law talks about the color of the emitted radiation. It depends on the frequency, and this frequency depends on the temperature of the heated body, whatever its composition.

Kirchhoff’s second law introduces the concept of a black body. For him, the blackbody is an excellent emitter of radiation, and all radiation generated in it is emitted.

As the black body presents easy practical realization, it has become fundamental for the study of thermal radiation. This is because the black body is characterized by an opening in a hollow object, which allows the reflection on the inner walls of any type of emitted radiation, thus absorbing this radiation.

The experimental setup, made from the radiation emitted by the black body, obtained graphs that were formed by the power of this radiation, but were not capable of being explained. This is because classical physics did not have the necessary information capable of obtaining a mathematical function that would give rise to such graphs.

Around 1900, the German physicist Max Planck decided to start from the graphs generated by radiation to finally arrive at the equation. Then quantum mechanics was born .

Through so much study, it is now known that every object that is superheated emits visible radiation, analyzed through the spectrometer (a device that scatters radiation).

Through this object it is possible to measure the length and intensity of electromagnetic waves, components of the emitted radiation, which allows the construction of its spectrum.

Ideal Radiator – A black body whose radiation depends only on its temperature.

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