Wien’s displacement law
 

The second important law concerning the black body radiation curves is that known as Wien’s displacement law, proposed in 1896, which states that the product of the wavelength at which the maximum radiation is liberated and the temperature of the black body is a constant. This may be expressed by
λ_maxT = constant (1)
the numerical value of the constant being equal to 2·90 × 10⁻³ m K. Wien’s law, therefore, provides a means of determining the temperature of a body at any distance without measuring the absolute amount of energy that it emits. By investigating the spectrum and assessing the wavelength at which the maximum of energy is being liberated, knowledge of Wien’s constant allows calculation of the temperature of the body. In astronomy, however, this method is limited to a fairly small range of temperature for which λmax occurs in the visible part of the spectrum.