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I was tearing open individually-wrapped cough drops in the dark, and noticed that when I pulled apart two adhesive-joined pieces of silvery plastic film, small flashes of light were produced. It was not a reflection--it was pitch dark.

What causes this?

The product is Bee M.D. Organic Honey Throat Drops--kind of gooey, old ones (laugh). They contain menthol and eucalyptus oil. I did some more experimenting and think the the flash could actually be separating the plastic from the cough drop itself rather than pulling apart the adhesive. The item is small, the room is dark, my finger has a wound, and all in all it's hard to tell for sure.

I don't know if this is a physics or chemistry question--it depends on the answer.

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  • $\begingroup$ It would be chemistry,-- a similar thing happens when you chew wintergreen lifesavers. Iirc, the oil of wintergreen (methyl salicylate) crystals are ruptured, accompanied by an energy transition in the visible range. So we get a flash. A similar thing must be happening here; except there are millions of different adhesives so it will be hard to find an answer unless you specify. $\endgroup$ Mar 4, 2012 at 7:31
  • $\begingroup$ I know of the wintergreen phenomenon; this one seems different. $\endgroup$
    – ErikE
    Mar 4, 2012 at 8:13
  • $\begingroup$ Its the same, see examples section of the wiki page linked below. $\endgroup$ Mar 4, 2012 at 8:16

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Some digging revealed the answer. It's called Triboluminescence. Basically, when certain materials are subjected to mechanical shock, chemical bonds are asymmetrically broken. This creates a charge separation, which on recombination ionizes the nitrogen in the air. The book Solutions of Fundamentals of Physics by Resnick-Halliday-Walker states in Volume 2, Chapter 22 Electric Fields (the 'chapter conundrum'), that nitrogen emits UV light after ionization. This UV light is absorbed by oil of wintergreen crystals in wintergreen lifesavers, which then emit blue light. All this absorption-emission of light is due to electrons jumping energy levels.

The examples section of the linked Triboluminescence wiki article mentions adhesive tape, so adhesive tape must have a similar mechanism. They're rather vague on the mechanism, though.

Another mechanism I've heard of is that the nitrogen in the air attaches to the broken bonds, releasing energy. That works, too.

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  • $\begingroup$ Can you be more specific about the "nitrogen ionization", i.e. which energy levels are involved? Is blue just a "spill-over" from a primarily UV event? Thx. $\endgroup$
    – Art Brown
    Nov 24, 2013 at 20:54

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