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Name: Peter O.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: Thursday, August 22, 2002

What causes infrared photons to be emitted from hot objects as thermal radiation? Can it involve an electronic transition following energy absorption by one molecule colliding with another?

The atoms in a hot object vibrate, but with a distribution of energies, i.e. frequencies of electromagnetic radiation. These movements involve rotations, vibrations, and changes in the electronic state of the atoms. This radiation is quantized (Planck blackbody radiation) but the "levels" are so smeared out and dense (i.e. the energy difference between levels) that it appears continuous and cannot be resolved. It turns out that as a result of the above, that all radiant bodies, regardless of their composition, have essentially the same distribution of radiation energies. The distribution depends only upon the temperature of the object. The theory (and experimental verifications of the theory) predict(s) that a significant amount of the emitted radiation is in the infrared region of the electromagnetic spectrum, where we experience radiant "heat". The change in the distribution of energies can be observed by heating an object. At low temperature we don't experience any heat, then as the temperature increases we sense that the object is hot, but still invisible to the eye. As the temperature increases the body glows a dull red that becomes brighter and more orange and then yellow. At even higher temperatures the body will appear to be bluish and then white because the emitted radiation in the visible is essentially the same for all wavelengths that the eye can see. Newton's experiment with a prism showed that sunlight, which is approximately "white" is composed of all visible wavelengths from the red (~700nm), to orange, to yellow, to green, to blue, to violet(~400nm). An exception is a gas at low pressure which also exhibits "line spectra" (i.e. very specific frequencies are emitted and/or absorbed). This is evident in the spectrum of the Sun at high resolution where the sharp absorption lines of individual atoms of gases absorb specific wavelengths. This is superimposed on the continuous spectrum, but cannot be observed except with more sophisticated spectrographs.

Vince Calder


Although I would just call it an "electron transition", you hit the nail on the head. All objects with some temperature have molecules bouncing around. Sometimes, a collision gives an atom enough energy to push an electron into a higher energy level. When the electron drops back down, it emits a photon. Hotter materials have more collisions. More photons are emitted, and at higher energies.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

Infrared radiation is very low energy and does not involve electronic transitions . Instead, it is the result of vibrational or rotational transitions, or even transitions in translational states of motion.

Greg Bradburn

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