A special kind of a body is the one which absorbs all kinds of radiations. This special body is called a black body.
Also, the radiation emitted by a black body comprises of radiation of all wavelengths. The wavelength at which maximum energy per unit time is emitted has a special relationship with the temperature of the blackbody. The product of wavelength and the temperature of the body is a constant. The equation is
λmax is the wavelength corresponding to maximum intensity (energy emmited per unit time per unit area),
T is the temperature of the black body,
b is a constant with a value of 2.898 * 10-3 m°K
This is called the Wien's displacement law.