Black light makes fluorescent powders glow
Q:
Why, under a "black light", do white objects appear to be illuminated?
 A:
The "black light" makes phosphors glow. Only objects with phosphor will appear
to be illuminated. A white T-shirt or white socks may glow, not because they're
white, but rather because they were laundered with detergent that contained
phosphors.
Glow-in-the-dark hard hat. Photo courtesy of
the US Department of Labor.
Phosphors are fluorescent powders
that, if you expose them to ultraviolet light, absorb the light energy and
immediately re-emit some of it as visible light. This process is called
fluorescence.
A black-light bulb is a
fluorescent tube that generates ultraviolet light similar to a normal
fluorescent lamp with a fluorescent layer coating the inner side of the bulb.
The low-pressure mercury vapor present in these lamps produces short-wave
resonance radiations at about 185 and 253.7 nanometers, says Mazime Gendre
of the Technical University at Endhoven in the Netherlands.
The phosphor helps convert these harmful radiation frequencies to "black
light" centered around 360 nanometers. These lamps produce some visible
light, too, in the violet, blue, green, and yellow part of the spectrum.
We would see this light, of course, and the light would not be black.
However, a bulb made of wood glass (glass containing nickel oxide) blocks the
visible and only transmits the black-light frequencies, says Gendre.
Our eyes see the colors in a
rainbow from red to violet, but not as high as the black-light frequencies. So
the light seems black.
Some of the phosphors in laundry
detergents stick to laundered clothes, like a white T-shirt. Shine a black,
ultraviolet light on the laundered shirt and its phosphors convert the
ultraviolet light to visible light. The white T-shirt glows in the converted
light. Many detergents now have little or no phosphor in them so the white
shirts remain dark or, at best, dimly lit--unless, of course, dipped in
fluorescent paint.
(Answered by April Holladay,
science correspondent, June 27, 2001)
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