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Q & A: Why does a black light make objects glow?

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Q:
Why does a black light make objects glow in the dark and change color?
- santos beltran (age 13)
nightingale elemaentary, chicgo,il.,u.s.a.
A:
Santos,

The colors of light that the human eye is able to see range roughly from red to blue in color. Blue light has a higher frequency than red light. The light which has frequency just lower than red light is called "infra-red", and the light which has frequency just higher than blue light is called "ultra-violet".

Both infra-red and ultra violet-light are beyond our eyes' range for efficiently detecting, however they are still very important. Infra-red light is often used to warm things (like heat lamps over food in restaurants), and ultra-violet light is the subject of your question.

A "black light" is just a light bulb designed to emit ultra-violet light. The reason these are called "black" is that if you look at the actual bulb it does not seem very bright (sort of a dim violet color), and if you put a black light in a dark room it really does not brighten it very much...the room remains almost black. These bulbs do emit lots of light however, itís just that we canít see it.

Some materials have the special property that they absorb ultra-violet light and then re-emit the light at lower frequencies that our eyes CAN see. This is called "fluorescence". These materials are sometimes found on our t-shirts, jackets or shoes, and when we walk near a black-light they will seem to "glow" since they are translating the invisible ultra-violet light into easy to see colors, most often white.

If you have a black-light handy, why donít you do the following experiment: Try putting different kinds of materials near it and make a list of which ones glow brightest and what color you see. Once you do this, tell us your results (use this web page again) and will post them for everyone in the world to see. Some of the materials you might try are different foods and drinks, plastics, bleach (ask your mom or dad first), dirt...whatever you can think of.

Let us know how it works, MS

(republished on 07/28/06)

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