Infrared and heat

[bassplayer142]

How exactly is infrared associated with heat? Is it just that infrared waves are emmited out of our body and when they contact something they turn to heat rather then being reflected?

[f95toli]

All bodies radiate energy. The frequency (or, equivalently, the wavelength) depends on the temperature.
This is described by Planck's law

http://en.wikipedia.org/wiki/Black_body_radiation

It turns out that a body held at a temperature of about 300K (i.e. room temperature) will emitt radiation with wavenlengths that are in the infrared region of the EM spectrum (the maximum will be at a wavelength of a few micrometer).

This also means that you can measure the temperature of a body by measuring its spectrum.