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Black Light

I was asking a group of energetic 6th graders what questions they had about science. I will detail more of the questions in a later blog but one in particular stood out. One young man with a father deployed overseas asked why a black light flashlight was used to find scorpions. This was an amazing starting point for a lesson on light and energy.

First I have to admit that I did not know black light is used to find scorpions but apparently, like white copy paper, some detergents and those amazing posters form the 60’s, scorpions fluoresce under black light. Black light is UVA light. You are familiar with UVB as it is more harmful and we try to minimize our exposure to sunlight to avoid too much UVB. Anything containing some amazing chemicals called phosphors fluoresce when exposed to UVA.

The spectrum is fascinating and consists of all kinds of energetic waves from the long radio waves on one end of the electromagnetic spectrum to the high energy short gamma rays at the other. Visible light is somewhat in the middle and consists of the rainbow of waves from red to violet. We all learned ROY G. BIV in school to recite the colors of the visible spectrum in order (red, orange, yellow, green, blue, indigo, and violet). There are amazing sites to help teach this. These are among my favorites:

Now phosphors are substances that emit visible light in response to being hit with some sort of radiation. In the case of black light these phosphors get hit with UVA (black light) and convert that UV light to visible light. This is why a white sheet of paper looks like it glows under black lights. You may have had a stamp with an ink that contained phosphors at a park or concert and when you wanted to return they would simply pass your hand under a black light to see the phosphor in the ink and the design to let you back in. Apparently scorpions have their own built in phosphors.

Many detergents we use to wash our clothes contain phosphors. That allows the clothes to “look whiter”. It really shows up under black light. If you take a white t-shirt or a handful of detergent you can see that under black light they almost seem to glow white.

There are hundreds of interesting uses for this phenomenon of making what seems to be invisible visible. Our new paper currency designs in the United States contain phosphorous dyes, newer paints have phosphors allowing some art dealers to determine the approximate age of a painting, phosphor containing dyes are injected into machinery to detect leaks under black light, and my favorite is forensics use of black light to locate blood and body fluids that contain phosphors. Several uses can be found at these sites:

This simple question is a doorway to a few really big ideas about matter and energy. At the high school we teach how electrons help produce light and how energy is measured and used. An understanding of the physics of waves allows students to understand everything from how cell phones and radios work to why we put on sunscreen. Energy and light comprise a huge part of chemistry and physics. At the elementary level it is great to begin an investigation about what kids know about light and expand their field of knowledge to know that not all energy can be seen. The visible part of the spectrum is only a small part.

Lots of highlighters contain phosphors and it will be fun to go back with a simple black light and allow the students to examine common things in their room to see which contain phosphors. Fixtures are available for under ten dollars or you can simply buy a black light bulb and use it in a lamp or trouble light. The military uses black light bulbs in flashlights and they indeed do use them to check bedding and tents and vehicles for scorpions. Not all scorpions light up under black light but it is a little comforting to this young 6th grade guy missing his Dad and for me to know that science is helping protect our troops.

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