Wien's displacement law states that objects that absorb and emit radiation have radiated energy curves that are similar, but peaks at different wavelengths, depending on their temperatures; the wavelength of the maximum energy decreases as the absolute temperature of the object increases and vice-versa. Wien's law is useful for using the visible color of radiant objects, such as stars, to determine the temperature of the object.
The product of an object's peak wavelength and temperature is constant across all wavelengths and temperatures. This constant, named Wien's displacement constant, is used to either determine the wavelength from a known temperature or the temperature from a known wavelength. Hotter objects emit their radiation at shorter wavelengths and appear blue since blue is the color associated with low-wavelength visible light. It is for this reason that cooler objects appear red, as their radiation emits at longer wavelengths. It is also important that the radiation curve is plotted as a function of wavelength because if plotted as a function of frequency or another variable, the peaks of the energy curves would be different. Wien's law also gave rise to the claim that the human eye's sensitivity evolved to be close to the peak emission of the sun. By taking the average temperature of the sun's surface and dividing it by Wien's displacement constant, the resulting wavelength is found to be near the peak sensitivity of human eyes.