You and Your Students!
Vicki Cobb, Education World Science Editor
Discover which states have secrets on their license plates
Physical Science, Optics
Setting the Scene (Background)
Some car thieves produce counterfeit license plates. The counterfeits might look authentic to ordinary people, but police know how to detect fakes. Many license plates have secret images that are not seen by the casual observer. Now you and your students will possess some inside information on a new way of looking at license plates.
Do this activity while you're on a school field trip, or make a special trip to the school parking lot. You will have to demonstrate for students how to look at license plates, but they will instantly want to try.
Stand facing a license plate four to eight feet behind the car. Walk slowly toward the car looking at the center of the plate. When your line of sight is 30 degrees to the center of the plate you will see a row of grayish symbols appear. If you get closer to the license plate, the secret symbols will disappear.
The secret symbols are invisible unless you view a license plate from an angle of 30 degrees. (The distance you need to be depends on how tall you are. Students will need to be closer than an adult will need to be.) The secret symbols are usually placed in a vertical row in the center of the license plate.
I can easily see the symbols on my New York plates during the daytime, but they are much more dramatic at night when I hold a flashlight next to my head directed at the same angle as my line of sight.
Many states currently use the process that embeds a "secret symbol" on the license plate, but all states' plates are not produced in this way. Your students might want to survey license plates for the states that surround their own. Which states use the process that hides a secret symbol and stops thieves?
Behind the Scenes
The secret symbols are hidden in the reflective sheeting that covers the plate. The sheeting has a layer of tiny glass beads embedded in a clear plastic. Behind the beads is a mirror-like coating. When viewed from most angles this coating reflects back just about all the light that strikes it. That's what makes license plates appear bright when headlights strike them.
The beads in the pattern of the secret image have a tiny dot on their surface that blocks the light from reaching the reflective surface behind them. So no light reflects from these beads when they are viewed from an angle of 30 degrees. These non-reflecting beads make up the darker areas of the secret pattern.
The reflective sheeting with the angled image is produced by 3M Corporation. More than thirty states use it on their license plates. The manufacturing process is so sophisticated that it is unlikely that crooks will be able to duplicate it.
The secret images on license plates serve the same purpose that holograms on credit cards serve. They make counterfeiting difficult or impossible.
Article By Vicki Cobb
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