Distribution energy Law of Wein

Stefan’s law deals effectively with the total amount of heat radiation comprising all wavelengths from a black body at a given temperature. It says nothing about the distribution of this energy of total radiation among the different wavelengths at different temperatures. This distribution is by no means uniform among the different wavelengths.  We now propose to examine this. It will be recalled that we donate the rate of emission of radiation per sq. cm of a body between wavelengths λ and λ + dλ at a given temperature T by E_λ dλ. We shall now proceed to investigate how E_λ of a black body on λ and T.

In a general, qualitative manner, this dependence is fairly obvious from the gradual change in colour, exhibited by a black body, as it is continuously heated. It appears dull red at 525℃, the Draper point, changing successively to cherry red at 900℃, orange red at 1100℃, yellow at 1250℃ and white at about 1600℃. This clearly shows that the radiation from it goes on progressively enriching itself in shorter wavelengths.

In other words, with a rise in its temperature, the maximum intensity of emission from a black body shifts towards the side of the shorter wavelengths, i.e. the maximum energy of emission gets associated with increasingly shorter wavelengths.