Physics Question #2366

Dan Hurtado, a 34 year old male from Tallahassee asks on November 17, 2004,

Can you explain the process by which hot materials emit light? Particularly, why is infrared light associated with heat, (especially since infrared is an arbitrary label given to a band of the spectrum?) Are higher temperatures required for higher frequency portions of the spectrum? Is that why naturally occuring X-ray sources are so rare?

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The answer

James Hope, A-level student, England answered on April 17, 2005

They emit light because, when you heat something up you are giving it energy. Most of this energy becomes kinetic energy of the particles (i.e. making them move faster) however some goes into the electrons. When the electrons receive energy, they take a "quantum leap" up to a higher orbital of greater energy. The atom cannot stay in this "excited" state, and so the electron must drop back down to its original orbital energy level. In doing this it must release the energy. It does so by emitting a photon. This photon is normally infra-red, or to the red end of the visible spectrum, because the "energy gap" between the orbitals (i.e. the energy released by the electron in the form of a photon) happens to correspond to frequencies around the red/infra-red part of the electromagnetic spectrum. You don't really get higher frequency photons being emitted, because at those energies the atom would get completely ionized (i.e. stripped of all electrons) before an electron orbital energy gap could get that big.

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