Energy distribution in black body spectrum

The first ever study of the infra red spectrum was made by Langley. He examined with the help of a spectrometer, the spectrum of sunlight, produced by a prism of rock salt. He focused the rays by means a lens on a bolometer arranged in a Wheatstone’s bridge, adjusted for zero deflection.

The black body spectrum, however – and, in particular, Wein’s displacement law concerning it – was experimentally investigated later by Paschen, Lummer and Pringsheim and Rubens and Kurlbaum during the years 1892-99. Here, we shall confine ourselves only to the experimental work of Lummer and Pringsheim, undertaken by them just two years after they had succeeded in verifying Stefan’s law.

After having first tried the various types of uniformly heated enclosures, they used in their final experiment in 1903 a specially constructed radiating chamber or black body, which came to be known by their name, and which, essentially consisted of a carbon plug at the middle of an electrically heated and effectively thermally insulated carbon tube, through which a slow stream of nitrogen gas was kept running, as a safeguard against its quick oxidation.

The radiation from the black body was rendered into a parallel beam by means of a concave mirror. It was then allowed to fall on a prism of fluorspar, placed on the turn-table of the spectrometer and thus resolved into a spectrum. This spectrum was then again made to fall which focused it on to a linear bolometer, consisting of a single strip of platinum foil, 0.6 mm in width. So that, by slowly rotating the turn-table, any narrow band of wavelengths desired could be made to fall on the bolometer. The bolometer. The bolometer was enclosed in an air-tight case to minimize absorption due to water vapour and carbon dioxide.

It will be noted that concave mirrors were used instead of lenses to avoid absorption of radiation by the latter. Also, the prism used was of fluorspar, firstly because it absorbed none of the radiation within the wavelengths being examined and secondly thereby enabling the prismatic spectrum to be converted into a normal one.