If $\lambda_m$ denotes the wavelength at which the radioactive emission from a black body at a temperature T K is maximum, then :-

Wien's displacement law: $\lambda_m T = b$, where $b = 2.9 \times 10^{-3}$ m K is Wien's constant.
So $\lambda_m = \dfrac{b}{T}$
$\Rightarrow \lambda_m \propto T^{-1}$
As a body gets hotter, the peak of its emission shifts to shorter wavelengths (e.g. red hot to white hot).