To understand the reason this experiment works, we first need to discuss black lights. Black lights earn their name because they give off very little light that can be seen by us. Light in the visible spectrum ranges from red, orange, and yellow, to violet.
Our eyes cannot detect light that exists beyond violet light in the visible spectrum. This light is called — you guessed it — ultraviolet light! This activity brought to you in partnership with Science Buddies. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Materials Tonic water Clear, plastic, disposable cup Measuring cup optional Medicine dropper Bleach Ultraviolet "black light" bulb This can be purchased at a party supply store or some hardware stores.
A room that can be darkened Preparation When handling the bleach, be sure to read and follow all safety precautions listed on the container. Adult assistance is required when handling bleach. Do not drink the bleach or the tonic water mixed with the bleach, and be careful to not get it on your skin. Avoid looking directly at the ultraviolet "black light" and shining it on your skin because the light can damage your eyes and skin.
Procedure Pour about one cup of tonic water into a clear plastic, disposable cup. In a darkened room turn on the ultraviolet black light and shine it on the cup. What happens to the tonic water in the cup when the black light shines on it? While still shining the black light on the cup of tonic water, use the medicine dropper to carefully add two drops of bleach to the tonic water.
Carefully mix the bleach in with the tonic. What happens when the bleach is added to the tonic? What happens after the bleach is mixed in with it? If you do not see a change in the tonic water, try adding and mixing in a few more drops of bleach. After dusting the fingerprints, either shine the black light directly onto the object or lift the prints using clear tape and paste the strips of tape onto black construction paper. Can students create a crime scene in which black-light evidence can help solve the crime?
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