Ask a Physicist: Phone Flash Sharpie Shock!

Lexie and Xavier, from Orlando, FL want to know:

“What’s going on in this video? Our science teacher claims that the pain comes from a small electrical shock, but we believe that this is due to the absorption of light. Please help us resolve this dispute!”

Well, I guess this is the kind of mischief that kids are getting up to these days. Still, it’s better than half the stupid “tricks” I remember hearing about in my student days. (Yeah guys, let’s play the “choking game“. That sounds like a blast.)

For those of you not in a position to watch the clip, the “trick” goes something like this: the person colors a black square on his arm using a permanent marker. Then, the flash “bulb” LED of a phone is held against the square as a picture is taken with the phone. The bright flash is accompanied by a sharp, stinging pain on the victim’s skin, they jump a little, and laughs are had by all. (Note: if you elect to try this at home, you will probably hurt yourself, seeing as it is a process literally designed to induce pain.)
Right off the bat, I’m immediately inclined to agree with Lexie and Xavier’s assessment—that it’s a light-based phenomenon. The phone’s flash LED is designed to put out a large amount of energy as light in a very short time. Since black-colored objects absorb visible light of any wavelength, it’s not surprising that coloring your skin black and mashing it against a high-power light source might transfer a good bit of that energy to your skin, resulting in a minor burn.
BUT, in the interest of fairness to the teacher, it’d be best to settle this like true scientists—with an experiment!
First, it’s important to figure out the details of your hypothesis vs. your teacher’s. From your point view, it’s obvious why the black ink is necessary for the trick to work on a lighter-skinned person—to absorb the light of the phone’s flash and convert it to heat. What role does your teacher think the ink plays? If he thinks it’s electrically conductive, perhaps using a silver sharpie—which ought to conduct electricity just as well, but which would reflect the light rather than turning it into heat—would settle the matter.
What other experiments could you design that would conclusively rule out one hypothesis or the other?

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