Introduction to Blackbody Radiation Spectrum - Civil Engineering (CE) PDF Download

Introduction to Blackbody radiation spectrum
Emissive Power:
The amount of radiation emitted per second per unit area of a surface is called emissive power of the surface. It is denoted by E. The unit of emissive power is Wm⁻² 

Absorptive power:
It is defined as the ratio of the amount of radiation absorbed by the surface in the given time to that incident on it during the same interval of time.It is denoted by a. It has no unit.

Blackbody:
A blackbody is one which absorbs all the radiations which is incident on it(Note that absorptive power of the blackbody is 1). Since a perfect blackbody cannot be realized in practice, a body which absorbs radiations in the visible region together with the radiation in the ultraviolet and infrared regions is considered as perfect blackbody. A good absorber of radiation is also a good emitter of radiation. Thus, when a blackbody is kept at high temperature compared to its surroundings it emits radiation in all regions.

 Spectrum of blackbody radiation
Lummer and Pringshein studied energy distribution in the spectrum of blackbody radiation at various temperatures. The results are depicted in the graph shown. The observation of the curves reveals the following facts:

1. Energy distribution is not uniform. Energy emitted by the blackbody increases with wavelength, becomes maximum (E_m) for a particular wavelength λ_m and then de-creases with the wavelength.

2. Total energy emitted by the blackbody increases with the increase in temperature simultaneously the wavelength λ_m corresponding to the maximum energy E_m emitted by the blackbody decreases with the increase in temperature. This is called Wein’s displacement law. Mathematically, λ_mT=Constant.

3. Total energy emitted by the blackbody is directly proportional to the fourth power of the absolute temperature. This is known as Stefan’s Law. Mathematically, E = σT⁴

Where σ is called Stefan’s constant which is equal to 5.67 × 10⁻⁸Wm⁻²K⁻⁴ .