Infrared (IR) includes EM waves between 780nm and 1mm in wavelength. (Source) As an object gets hotter, it emits a greater amplitude of infrared. What determines the particular frequency(s) of IR an object emits? The temperature, and is independent of the object itself? The molecular makeup of the object itself, and hence is independent of the temperature? Or does it depend on both?
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If an object is in thermal equilibrium, then it will be emitting on all frequencies, but the intensity of emission depends on frequency. This dependence is given by the Planck's law, which has a peak, with most of radiation falling around this peak. The peak frequency is proportional to the temperature and in everyday conditions usually falls in infrared.
(In the figure below the intensity of the radiation is shown as a function of the wavelength - the higher the temperature the shorter is the wave length corresponding to the peak, the more the "color" of object shifts towards blue.)
Related: Why are only infrared rays classified as "heat rays"?
Roger V.
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