Interesting way of putting this question! So called "black" light
consists mostly of light waves of a higher frequency (shorter
wavelength) than our eyes are sensitive to. Another name for this kind
of light is "ultraviolet", because violet, or purple, light is the
color of light we can see that has the highest frequency, and adding
the "ultra" to it means we've gone off the edge of our visible range
into something else we cannot see.
Most commercially available lightbulbs which produce "black" light
produce some purple light along with the ultraviolet light, so they
don't actually look black.
Light that has too low a frequency (long wavelength) to be seen is
called "infrared" light for similar reasons -- it is off the red end of
the visible spectrum. For some reasons, infrared lights are not
referred to as "black" lights although they are equally invisible. Even
lower frequencies (longer wavelengths still) are microwaves, cell
phones, television, and radio broadcast waves.
When light is absorbed, it transfers energy in little clumps
called 'photons'. Ultraviolet light has a lot of energy per photon,
more than in the photons of visible light. When ultraviolet light
strikes some materials, the energy is absorbed by the electrons in the
atoms of the materials and re-radiated at lower frequencies. This
process is called "fluorescense," and is the reason why fluorescent
stuff looks so interesting when ultraviolet light shines on it. What
looks like dim, purple light shining on an object will make the object
glow a bright color, and that color depends on the material. Infrared
light doesn't do that because each infrared photon has less energy than
a visible photon (although there are special materials which can double
the frequency of an intense beam of light).
White light is a combination of light of all colors of the visible
spectrum. Adding invisible infrared or ultraviolet light to white light
does not change its color (the sun emits all of these kinds of light).
A rainbow or a prism can split white light into its colored components.
A suggestion on doing controlled experiments -- because you cannot
see most of the light coming out of a "black" light bulb, it is hard to
tell just how much light there is -- is it a dim black light or a
bright one? Even using bulbs of the same wattage may not give the same
amount of light energy -- one may heat up more than the other. You may
need to find the "efficiency" of the bulb tabulated somewhere, or some
measure of the light output of the bulb. Also, be sure to use a similar
kind of fixture -- comparing a long tube bulb with an incandescent
pear-shaped bulb may introduce differences because the shape of the
reflector behind it will be different.
And remember- the ultraviolet light can be quite destructive to
your eyes even though you can't see it, so use special ultra-violet
protective glasses when working with it. Really powerful black lights,
like those used in tanning salons, can cause skin damage, including
cancer. We suspect that your lights will not be that intense, but do
watch for signs of sunburn.
Tom (and Mike)
(published on 10/22/2007)