Physics

Overview of Black Body Radiation Theory



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The black body radiation is a key concept of the physics of the 19th century.  Its importance stems from the fact that it led to the development of the current quantum theory of light.  The black body is so important in physics as an available model that its investigation mimics the investigation of the unavailable systems of remote stars in the universe.  Black body radiation mimics the radiation from the surface of very remote stars.

Black body is a body that is a perfect absorber of light and yet it is a perfect emitter of radiation as well.  Stars are gaseous spheres that have very high temperature that emit electromagnetic radiation.  Therefore, this analogy between a black body and stars made them a convenient available method of investigation.   Some of the electromagnetic radiation that the star emits comes from the nuclear conversion in the star itself of hydrogen atoms to helium.  So the emission of radiation by the stars is not purely due to its high surface temperature. 

The emitted radiation from a perfect black body depends only on its temperature and not on its chemical and physical structure.  Non-ideal black body differs in its radiation from that of a perfect black body because of the dependence on the chemical and physical structure of the non-perfect black body.  Black body radiation was investigated by many physicists in the 19th century and its interpretation led to problems that are manifested by their being data that could not be interpreted based on classical laws of electromagnetism. 

A physicist by the name of Wien investigated the radiation of the black body and formulated a physical law that relates the color of the emitted radiation as a function of the temperature of the black body.  This law states that everybody with a temperature that is not zero degrees Kelvin emits electromagnetic radiation.  The higher the temperature of the black body the shorter the wavelength of the intensity of the emitted radiation.   Wien law can be used to estimate the temperature of various stars based on their color.  Wien law enables us to determine the color of a black body as a function of its temperature. 

Other scientists, notably Joseph Stephan formulated in 1789 a law that relates the temperature of a black body to its emitted radiation.  This law is called Stephan-Boltzman law and has the following form:

E=sigma*T*T*T*T

A star is a gaseous sphere whose surface behaves like a black body.  Investigating the distribution function of a black body led to the development of the quantum theory of light by formulating mathematical terms that use quanta of light in the distribution function of the electromagnetic radiation emitted by a black body. 

Stars are not considered an ideal black body due to the presence of atoms that absorb light instead of radiating it.  

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