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Note, also, that heat and infrared light are not the same thing. Heat is a relatively arbitrary measurement of the kinetic energy of an atom or molecule, typically caused by the absorption of EMR. If we were to look in a vacuum chamber while wearing our Quantum-Vision Goggles, we would see EMR, but we would not see heat; heat does not exist in the absence of matter.
Thus, when an object absorbs and emits EMR, it is not distributing the emitted energy arbitrarily between visible light and heat. All of the energy is transmitted as EMR, but some of that EMR becomes heat upon absorption by other matter. Infrared light is not visible to humans, and thus its presence is communicated to us via the heat that it produces.
Atoms consist of a nucleus that has neutrons and protons. The electrons orbit around the nucleus of the atom. The distance at which electrons can orbit around the nucleus is not continuous; electrons can only orbit at discrete distances that are determined by the energy possessed by the electron.
Electro-magnetic radiation is energy. The energy in a ray of light is quantized as photons with the energy in a photon given by E = h*f where h is the Plank’s constant and f is the frequency of the ray of light.
Visible electromagnetic radiation has a higher frequency than infrared light and this also makes the energy in a visible light ray more than that in an infrared light ray. When atoms are exposed to electro-magnetic radiation it is not possible for them to absorb all the radiation, they can only absorb radiation in discrete frequencies that correspond to the change in the energy levels of the orbiting electrons. An atom can absorb a large amount of energy that instantly shifts electrons to levels that are far away from the nucleus. While returning back to their original orbits electrons do not have to release all the energy absorbed at once. Instead they can gradually return to the original level by emitting photons of light that correspond to the difference in subsequent energy levels at different orbits. This makes it possible for atoms to absorb a large amount of energy from visible light rays but emit infra-red light.
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