I am familiar with the usual concept that heat is kinetic energy (the vibration of matter particles) but I also read that heat is one of the faces of electromagnetic radiation, such as visible light, radiowaves or x rays. Is it correct? Is it actually both? (e.g. EM radiation just a side effect of the kinetical movement ? and probably able to cause kinetical movement again on the matter absorbing it?) if really EM radiation, in which lenght of the spectrum?