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define wien's displacement law

The energy distribution in the spectrum of a black body radiation shows that for any temperature T,there is one wavelength(?_m)for which the energy emitted by the black body is maximum.This wavelength ?_m is inversely proportional to the absolute temperature(T) of the black body i.e.,
                                                                         ?_m ?  1/T

                                                    or                ?_mT = constant = b

The above equation is known as Wien's displacement law.The value of constant b is found by experiment to be 2.9 x 10^-3 mK in S.I.units.Thus Wien's displacement law may be stated as:

The product of the wavelength( ?_m) at which maximum energy is emitted and the absolute temperature (T) of the black body is always constant.