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I know that the emission of light is evidence for a chemical reaction. However, are there any instances where emission of light can also be considered a physical reaction?

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  • $\begingroup$ Synchrotron radiation? Czerenkov radiation? Bremsstrahlung? What you are searching for? $\endgroup$ – HolgerFiedler Dec 16 '18 at 19:47
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    $\begingroup$ An Edison-style light bulb? LEDs? ;-) Actually chemical rections emitting light are rather rare, esp. if you take out those that simple become very hot. If you don't live in a rural area (fireflies) or have had a good chemistry teacher, you probably have never even seen a light that would qualify as "chemical". $\endgroup$ – Karl Dec 16 '18 at 19:57
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Most light doesn't result from chemical reactions

While there are many chemical reactions that can emit light and many related reactions where the emission is related to chemistry (for example the light from some flames is caused by electronic transitions), most light you see isn't.

An incandescent light bulb emit light because of black-body radiation produced because it is hot (this is not normally though of as having anything to do with chemistry). Fluorescent tubes emit light because of electronic transitions in gases or phosphors (both of which might be considered closer to chemistry because they involve electronic transitions but not the sort that indicate a reaction as they are entirely reversible). LEDs are similar.

A well set up Bunsen burner emits a small amount of light from the chemical reactions occurring in the flame but most well-regulated flames don't emit much light. A candle, on the other hand, has a relatively bright flame. but most of the light comes not from chemical emissions but from the incandescent emission from small soot particles produced from the (deliberately) inefficient burning. Ultimately that heat come from chemistry but most of the light comes from the basic black-body physics of hot objects.

So, in short, most light is physical not chemical.

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  • $\begingroup$ I was initially thinking about the light bulb as an example of light emission as a physical change. Because I know that electricity passing through the filament in a light bulb causes a physical change. Heat and light are produced, yet no chemical change. But the Introductory Chemistry textbook I have presents emission of light as evidence of a chemical reaction (example of glow sticks and fire flies) and no mention that it could also be physical change. But the light bulb made me think otherwise and, thus, I wanted to reach out to others for additional input/confirmation. $\endgroup$ – user72414 Dec 16 '18 at 21:09
  • $\begingroup$ There are a number of places where it might be appropriate to write "visible light" rather than just light. $\endgroup$ – Buck Thorn Dec 16 '18 at 22:21
  • $\begingroup$ @user72414 Can you give the name of this book, so others might be warned about it? $\endgroup$ – Karl Dec 17 '18 at 0:08
  • $\begingroup$ Introductory Chemistry (6th Ed) by Nivaldo J. Tro (pg. 209-210). It's interesting that the author does point out how gas evolution is not always an indication of a chemical change (physical change of liquid water to gas for example). The book also stresses that the changes only provide evidence but not definitive evidence (which is true). But I think it would have been less confusing had the author included something about how emission of light can be from chemical changes AND physical changes. I guess that will come about in the 7th edition?;-) $\endgroup$ – user72414 Dec 17 '18 at 1:14

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