Hello students, we are familiar with the mode of transfer of heat viz. conduction, convection and radiation. As we have studied in previous articles that conduction and convection requires material medium for transfer of heat which takes place in slow manner. The fastest mode of transfer of heat is with the help of electromagnetic radiation, known as radiation.
Radiation do not requires any material medium for the transfer of heat. Electromagnetic waves like ultraviolet waves, infra red waves are of different wavelength carries energies without necessity of any medium and hence heat transfer in vacuum is possible due to radiation.
Heat incident in the form of radiation on substance can be absorbed, reflected and transmitted by the substance. Amount of absorption, transmission and reflection of heat depends upon the nature of material of substance. White coloured material are considered as good reflectors of heat and black coloured materials are considered as good absorber of heat.
In this article we are going to learn about the concept of black body radiation.
Let’s understand concept of black body radiation in details….!
Perfectly black body:
A body which absorbs all the heat radiations incident on its surface is called as perfectly black body.
In practice the perfectly black body cannot be realized. But the lamp black or the platinum black can be considered as a perfectly black body as they absorb more than
98 % of the incident radiant energy.
Spectrum of a black body radiation in terms of wavelength
In 1899, Lummer and Pringsheim performed experiments to study the distribution of energy with the wavelength in spectrum of black body radiations. The intensity of black body radiations corresponding to wavelength ranging from zero to infinity is measured and plotted at various temperatures of black body. The intensity verses wavelength graph is called spectrum.
Different curves are plotted for different values of temperatures of black body shown in fig.
Following are the conclusions that can be drawn from the graph,
At given temperature of black body
- The energy emitted is not distributed uniformly among all wavelengths.
- The energy emitted is maximum for wavelength λmax and it falls on either side of it. The wavelength is called wavelength of maximum emission.
With rise in temperature of black body
- The energy emitted increases rapidly for any wavelength.
- The wavelength for which energy emitted is maximum and shifted towards shorted wavelength side.
Important law related to spectrum of black body radiation…..!
1) Wien’s displacement law
The wavelength (λm) corresponding to maximum energy emitted by a black body is inversely proportional to its absolute temperature.
This law is used to find the temperature of the sun and stars.
2.) Stefan-Boltzmann’s law of black body radiation:-
The amount of radiant energy emitted per unit time per unit surface area of a perfectly black body is directly proportional to the fourth power of its absolute temperature. OR
The emissive power of a perfectly black body is directly proportional to the fourth power of its absolute temperature.
If T0 is the temperature of surrounding, then
∴ E = e σ[T4 – T04]