concepts in quantitatively understanding Radiation

let us start it from where we have left.as in earlier section we figured out that the bodies/objects tend to reflect energy or spectrums of energy which are unique to them selves,having that in mind let us know about the terms such as kinetic temperature,black body,emissivity and so on..

what do we mean by "temperature" ? , well many would come up with different ideas and definitions but let us say it in this way, simply temperature is a measure of the concentration of heat, more the heat is concentrated more the temperature.

we can measure the temperature of a body made out of material by simply putting the thermometer in contact with the body,the reading we get from this is called the kinetic temperature of the particular body ( i.e. The concentration of kinetic heat of a body of material)

in a previous article we skipped the topic of blackbody radiation, so what is exactly a black body ??
     if there exists a body which is made out of a material that absorbs all the radiant energy that strikes it.then such a body/material is called a black body, not only that but also it can radiate the maximum amount of energy and this energy is dependent on the kinetic temperature.
the radiant flux of a black body is proportional to the 4th power of its kinetic temperature, and that is mathematically expressed as

here Fb is the radiant flux of the black body Tkin is the kinetic temperature and the constant s which equates the two sides is called as the Stefan-Boltzman constant, and this equation is the mathematical representation of the Stefan-Boltzman law.

we have already stated that " if there exists a body...." and so on but does there exist such a body which can be called as a black body, NO... its just an abstract physical quantity, the real objects dont behave in that manner.so we take this hypothetical black body as an indicator to express a property called emissivity of a real existing body.
the emissivity(e) is the proportion of real radiant flux to the radiant flux of the blackbody

here Fr is the radiant flux from a real material

(for more about this image and the thermal emissivity goto thermal-imaging-blog )

as you can see the value e=1 for a black body and e<1 for all real material.as we already know the objects tend to reflect certain type of radiant enery more than the others so the emissivity of a material is different when is measured at different wavelengths of radiant energy.