# Infra red light.

1. Oct 31, 2008

### Monitor16807

So, I always thought that NIR was created by the motion of atom at they're everyday temperature, but light is an electrons which goes from one energy state to an other, so how does just "shaking" electrons make light?

2. Nov 1, 2008

### mathman

When an electron goes between energy states a photon is emitted or absorbed. Light is therefore emitted when an electron loses energy.

3. Nov 2, 2008

### cepheid

Staff Emeritus
Not all light comes from atomic transitions (which are decidedly quantum mechanical processes)

One way of looking at it is in terms of Maxwell's equations. A time-varying electric field induces a time-varying magnetic field and vice versa, so that these fields can be self-sustaining. Maxwell's equations also show that such fields are governed by a wave equation, which means that an allowable solution is an "electromagnetic wave" that "detaches" itself from its source and propagates through space, carrying energy and momentum with it. The question is how to generate the time-varying electric field in the first place: an oscillating charge will do that. So, to recap, a charge moving at a constant speed will just have a "static" field, but *accelerating* charges will radiate. Now, since the material in question has such "oscillators" over a very large range of energies, a broad range of frequencies (colours) is produced. (Look up blackbody radiation as well). We call this "continuum" or broadband emission, as opposed to the discrete spectra caused by transitions between atomic energy levels (line emission). Now, exactly where the peak of the distribution of frequencies occurs depends upon the temperature of the material (i.e. on how excited the oscillators are, on average). Therefore, while room temperature objects will emit most of their EM radiation in the NIR, much hotter objects will emit visible light (e.g. molten metal, or a hot lightbulb filament ;-) ). The moral of the story is that a LOT of the light we encounter in everday life is actually of this variety. If you didn't know anything about quantum mechanics such as "photons" or "allowed" atomic energy levels, then I think you'd assume that "shaking electrons around" is really the only way to produce EM radiation. In other words, this is a very classical way of looking at things. Exactly how to describe this so-called "thermal" emission in terms of the photon model is something I am not knowledgeable about.

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