Specific distribution in black body radiations

In this article we will discuss Specific distribution in black body radiations. It includes experimental results as well as graphical representation.

Experimental Arrangement :

Lummer & Pringsheim investigated the distribution of energy among the radiation emitted by black body at different temperatures.The experimental arrangement of Lummer & Pringsheim is shown in the figure below :
 

Specific distribution in black body radiations
Fig.1. Experimental arrangement of Lummer & Pringsheim

The radiation from the black body passes through the slit S1 and falls on reflector M1. The parellel beam of radiation falls on rock-salt or fluorspar prism ABC placed on the turn table of spectrometer after the reflection.
 
• The emergent light is focused by the reflector (concave mirror) M2 on a line bolometer placed behind the slit S2 .The bolometer is connected to a sensitive galvanometer. 
 
 The turn table is rotated slowly so that different part of radiation spectrum successively falls on the bolometer & corresponding deflections in galvanometer connected in bolometer circuit are read.
 
The intensity of each line is proportional to deflection in galvanometer.
Then a curve is drawn between intensity & wavelength. The distribution curves drawn for different temperatures are shown below :
 

specific-distribution-in black-body-radiation
Fig 2. Curve between intensity & wavelength
 
The curve shows that the :
 
i) The distribution of energy is not uniform in the radiation spectra of a black body.
 
ii) At a given temperature, the intensity of incident radiation increases with increases of wavelength and become maximum at particular wavelength.
 
iii) An increase in temperature cause a decrease in λm such that :
 
       λm.T = constant = 0.2896
 
where, λm = wavelength for which energy emitted is maximum.
 
T = Temperature
This relation is called Weins displacement law.
 
iv) An increase in temperature causes an increase in energy emission for all wavelength.
 
v) The area under which curve represent the total energy emitted by the body at a particular temperature for a range of wavelength considered.
This area increases with increase of temperature. That area is directly proportional to fourth power of absolute temperature.
                                 E   T4
 
This is known as Stefan’s law.

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