...are just under the visible spectrum, hence infra red (below red). Heatexchange happens through IR radiation. It's due to vibration or torsion of molecules (body, fire, temperature, ..)
Stars, dust between stars, Sun, Blackbody radiation, Heat
Heat seeking systems, electronic photography, remote controls or comm. devices (GSM, laptop, ..), fibre optics
Mobile phones, satellite communications, radar, TV, radio, ..
Interaction with matter: