|MadSci Network: Chemistry|
A "black light" or Ultraviolet lamp is almost the same as the flourescent lights used in most classrooms. The difference is that "UV" lights lack the white phosphor coating, and they are made with dark blue glass.
Flourescent tubes contain a near vacuum with a little mercury vapor. If you could find a flourescent tube made from clear glass with no white coating, you'd be able to see some tiny droplets of mercury rolling around inside. When high voltage is applied across the ends of the tube, a big, silent, fuzzy spark jumps through the low pressure gas inside, and this makes the gas glow. The gas gives off various frequencies of light, and humans usually see these as colors. Mercury gives off green, blue, violet, and invisible ultraviolet (UV) frequencies. If you point an inexpensive classroom spectroscope at the flourescent lights in the ceiling, you will see the green frequency emission as a thin green stripe (although the rainbow of colors coming from the white flourescent powder will make the blue and purple strips nearly impossible to see.) Other gasses give off other frequencies. For example, neon gives off strong red/orange, which gives neon lamps their bright color.
Mercury gas gives off green, blue, and UV light. If your black light tube was made from clear glass, the bright blue/green glow would light up the room. Because of this, black lights are made with dyed glass which blocks most of the visible light but passes the UV light. If a perfect dye were available, black light tubes would not glow purple when turned on, they would look totally black, even when turned on. But the dye in the glass lets a little violet visible light through as well as passing lots of invisible UV.
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