When a piece of sodium chloride is heated in a Bunsen burner flame the sodium atoms become excited and emit light at the wavelengths of 589.0 and 589.6nm. We all know this nasty yellow light from traffic lighting.
But the sodium atoms also absorb light at these wavelenghs. So if light is
sent through the yellow flame it's 589.0 and 589.6nm compounds are absorbed
and the flame casts a shadow at those wavelengths.
With" normal" light this isn't noticeable - no one misses these narrow
lines of the light spectrum. On the foto below light from a 35W halogen bulb
was sent through the flame - you can only see the shadow of the steel wire with
its tip covered with NaCl.
Now I interchanged the 35W halogene bulb with a 35W sodium vapor lamp:
Now the yellow part of the flame DOES cast a shadow! Additionally the cooler areas of the flame where the sodium atoms don't produce much light by themselves but still absorb light look dark.
If you compare both pictures again you can see that the dark areas of the sodium-illuminated flame aren't smoke or soot.
More "dark fire"...
