Special Distribution of Blackbody Radiation - Wien's Law
Wien's displacement Law
Wien's displacement law states that the black body radiation curve for different temperatures peaks at a wavelength inversely proportional to the temperature. λmax=Tb where T is the absolute temperature in kelvin. b is a constant of proportionality called Wien's displacement constant, equal to 2.8977729(17)×10−3mK
Spectrum of blackbody radiation vs wavelength at different temperatures
Intensity increases with temperature and peak wavelength decreases with temperature
Heat detection devices
Bolometer and Thermopile are two devices to measure radiation. Bolometer It is based on the working principle of the Wheatstone bridge. A thin foil of platinum is cut into strips in a grid like pattern. Four such identical grids are taken to form a Wheatstone bridge with a battery and a galvanometer connected. Grids 1 and 4 are allowed to fall radiation upon whereas grids 2 and 3 are protected from radiation. At balance condition the resistances of the grids are related as: R2R1=R4R3 where R1,R2,R3,R4 are the resistances of the grid. As temperature increases on radiation, R1 and R4 increases but the product R2.R3 remains constant. This brings about a deflection in the galvanometer which is a measure of radiation. Thermopile It works on the principle of Seeback effect. A thermopile s composed of a number of thermocouples made from bismuth and antimony in a series connection to increase the sensitivity of the system. The free ends are connected to a galvanometer. The junctions are arranged in such a way that all the hot junctions lie on a plane face and all the cold junctions lie on the opposite plane face.The face of the hot junctions is blackened and exposed to radiation while the other face is shielded. A metallic cone generally concentrates the radiation on the hot face. The radiation is measured by the deflection in the galvanometer.