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Q & A: Lights for Plants

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Most recent answer: 10/22/2007
I am doing a science fair project to see the efect of black light and white light on plant. I was wonering if you could give me some background on the 2 types of light....
- Jessica (age 16)
RCHS, Michigan
Hi Jessica,

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)

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