-: Black Body :-
A body which can absorb all radiation energy of all wavelength and no part of the radiation energy falling on the surface is reflected energy falling on the surface is reflected or transmitted is called Black Body . It is also called perfectly black body .
In other words , For a black body the absorption coefficient is unity . None of the bodies in the nature is perfectly black body . Lamp-black and platinum black which absorb visible and near infra-red radiations . The lamp-black absorbs about 96% of the incident radiations while platinum-black absorbs about 98%. These are very close to the perfectly black body but are not truly black bodies .
Black Body Radiation :- When a black body is heated to a suitable high temperature , it emits total radiation which is known as black body radiation.
-: Black Body Radiation Spectrum :-
or
Energy Distribution in Black Body Radiation
Lummer and Pringsheim obtained spectral distribution curves for black body corresponding to various temperatures ranging from 621K - 1642K . These curves are shown in figure, where all the curves have similar shapes-
1- The energy distribution in the radiation spectrum of black body is not uniform . As the temperature of the body rise , the intensity of radiation for each wavelength increases.
2- On increasing temperature the emissive power of the surface increases for all wavelengths . The emissive power is proportional to the fifth power of absolute temperature -
E⋌ ∝ T^5
3- With increase in temperature the wavelength corresponding to maximum intensity shifts towards the shorter wavelength .
4- The height of the peak also rise very fast with increase of temperature .
5- For a given temperature the total energy of radiation is represented by the area between the curve and the horizontal axis. And the area increases with increase of temperature being directly proportional to the fourth power of absolute temperature .
i.e.
E ∝ T^4
This is Stefan's Law .
A body which can absorb all radiation energy of all wavelength and no part of the radiation energy falling on the surface is reflected energy falling on the surface is reflected or transmitted is called Black Body . It is also called perfectly black body .
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| Image Source : Google |
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| Image Source : Google |
In other words , For a black body the absorption coefficient is unity . None of the bodies in the nature is perfectly black body . Lamp-black and platinum black which absorb visible and near infra-red radiations . The lamp-black absorbs about 96% of the incident radiations while platinum-black absorbs about 98%. These are very close to the perfectly black body but are not truly black bodies .
Black Body Radiation :- When a black body is heated to a suitable high temperature , it emits total radiation which is known as black body radiation.
-: Black Body Radiation Spectrum :-
or
Energy Distribution in Black Body Radiation
Lummer and Pringsheim obtained spectral distribution curves for black body corresponding to various temperatures ranging from 621K - 1642K . These curves are shown in figure, where all the curves have similar shapes-
1- The energy distribution in the radiation spectrum of black body is not uniform . As the temperature of the body rise , the intensity of radiation for each wavelength increases.
2- On increasing temperature the emissive power of the surface increases for all wavelengths . The emissive power is proportional to the fifth power of absolute temperature -
E⋌ ∝ T^5
3- With increase in temperature the wavelength corresponding to maximum intensity shifts towards the shorter wavelength .
4- The height of the peak also rise very fast with increase of temperature .
5- For a given temperature the total energy of radiation is represented by the area between the curve and the horizontal axis. And the area increases with increase of temperature being directly proportional to the fourth power of absolute temperature .
i.e.
E ∝ T^4
This is Stefan's Law .
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