If you want to think deeply about fire, Andres Tretiakov is your guy. The award-winning science educator, a technician at St. Pauls School in London, recently showed students this spooky effect—fire that absorbs light. For the demonstration, he dissolved table salt in ethanol and lit the vapors. Under normal illumination, that mixture burns yellow because of the characteristic emission spectra of sodium: Valence electrons in the sodium, excited by the heat, fall back to a more stable energy level or ground state, emitting almost monochromatic yellow light at 589 nm in the process. But the sodium also absorbs at that wavelength. Tretiakov placed a sodium vapor lamp next to the beaker, and the flames absorbed that light, becoming an eerie black to our eyes.
Submitted by Andres Tretiakov
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