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Jun. 10, 2011

Science Dad on Luminescence

by Vince Harriman

Click to enlarge images

Beckett waiting for a flash

As a parent, I often wonder (and worry) if any of these projects and experiments stay in Beckett's head. Yesterday, when he got home from camp, he told me that he had caught and identified a male Cabbage White butterfly. Now we haven't seen many Cabbage Whites this year and certainly haven't talked about them since last summer. Honestly, I didn't remember if the male had one spot or two, so I looked it up in his bug book and was very pleased to see that he was right. Science Dad didn't remember how many spots meant what, but Science Kid did!

Also this week we saw and captured our first firefly -- which was perfect because I had been planning some activities to talk about luminescence. We put the firefly in an Erlenmayer flask that Beckett had brought home from camp (they had a science day). Beckett pulled out the bug book (again) and identified it as a Big Dipper. We were less sure whether or not it was male or female: males have 6-9 abdominal segments and females have 6. Ours had six so we couldn't be exactly sure. We decided (for no other reason than Beckett names all his insect 'pets' Mike) that it was a male. We turned out the lights, watched it flash a bit, and decided to let it go. Fireflies use an enzyme called luciferase in conjunction with ATP and oxygen to produce light. The firefly mostly uses the light to attract mates, though here in North America there is a second firefly known as the Pennsylvania firefly the female of which uses its flash to attract male Big Dippers and eats them.

Glow in the dark stars

We started looking around to find other types of luminescence. Beckett started with a glow in the dark star he found in the closet. He placed it under a bright lamp for several minutes then took it into the bathroom to see it glow. Almost all glow in the dark products use phosphors in some form to produce light. Most use a compound called zinc sulfide -- this molecule absorbs the energy from light, stores it, and as a consequence, becomes energized and begins releasing this energy as visible light. Scientists have formulated hundreds of phosphor compounds capable of glowing, and these compounds are used in all kinds of toys, tools, and technology. Television and computer screens are examples. One exception is watches. While inexpensive watches use phosphor compounds and light to energize and give off light, some expensive watches use radiation as the energy source. The phosphor compounds absorb the radiation given off by radioactive compounds and release light after energizing.

Excited about glow sticks.

Next we looked at chemical luminescence or chemiluminescence. I brought home some glow sticks that I found in a dollar bin at a local hardware store. Glow sticks (or light sticks) use a combination of three different chemicals to produce light -- the first two chemicals are kept separate in the glow tubes within small glass capsules inside the plastic sleeves. Breaking the glass capsules mixes the chemicals together and causes a chemical reaction that releases energy. The energy is is absorbed by the third chemical (with the phosphor compounds) which then excites the molecules releasing light. While the chemical formulas are quite complex, understanding what is happening in the reaction is quite simple -- the chemical reaction of the first two compounds creates an energy source for the phosphors. This was a good time to remind Beckett of the law of conservation of energy -- energy can neither be created or destroyed -- it can only change forms. In this case, we were changing chemical energy into light energy.

Tonic water glowing under 'black light'

Finally, we pulled out a black light to see what that would reveal. A black light is usually a type of florescent light with either a very specific light range (produced by a single phosphor) or a regular light covered by a filter that blocks out all but a small band of Ultra-Violet light. Some black lights (the kind we had) are actually incandescent bulbs with a filter around them and are very inefficient. I had Science Mom pick up a bottle of Tonic Water when she was at the grocery store to put under our light. You can just see the top of the bottle is barely glowing a pale green. Tonic water has quinine in it, the main ingredient of which is composed of phosphors. You can get a better effect under a black light with highlighter pens, but we couldn't find ours for this project.

About Vince Harriman

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.

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