Loony Tuner

Starring

You and Your Students!

Script By

Vicki Cobb, Education World Science Editor

Synopsis

Make a tuning fork appear to wiggle weirdly.

Genre

• Physical Science
• Sound
• Human Body (Illusions)

Required Props

• tuning fork (your school's music teacher might have one)
• rubber band
• computer monitor or TV set

Setting the Scene (Background)

One of the problems of teaching science is how to make abstract ideas more concrete for students. Some of the phenomena that interest scientists -- like sound waves -- can't be seen. The source of sound is vibration, a rapid back-and-forth motion that is too fast to be seen by the human eye. A struck tuning fork's vibrations are seen as a blur. But you can use a computer monitor screen (or a blank TV screen) to slow down the vibrations and to demonstrate a peculiar visual illusion.

Stage Direction

You can probably get a tuning fork from your school's music teacher. (If you can't get one, a stretched-open rubber band is a good substitute.) Then, all you need is a blank TV screen or computer monitor. Do this activity as a demonstration first; then have kids work in pairs to experiment with it themselves.

Show-Biz Science is scripted by popular children's book writer Vicki Cobb. Click to learn more about Vicki or to read a brief synopsis of her philosophy of teaching science. Visit our archive of archive of Show-Biz Science Activities. Watch for a new activity each week. Then chat with Vicki -- share your feedback and ask your questions about teaching science -- on our special Showbiz-Science message board. Be sure to visit Vicki's Kids' Science Page for more great science fun, a complete list of her books, and information about how you can invite Vicki to come to your school. And don't miss her library of science videos too. Or visit Vicki and other great authors of nonfiction for children at the INK Think Tank.

Plot

Act I
Create a blank screen on the monitor by opening a new file. (If you use a TV screen, turn the dial to "video".)

• Hold the tuning fork by its handle. Gently strike it against a desk to make it vibrate. Look at it as the tone sounds. You can see that it's moving but the motion is a blur.
• Now move it in front of the monitor. Hold it so that the tines are horizontal -- the tines will appear to be vibrating slowly.
• Hold it vertically and you get a different effect -- the tines will appear to be distorted with a wave-like motion.

Act II
You can see a similar effect with a rubber band. Cut one open to make a rubber string. Have a student stretch it tightly and hold it in front of the monitor. Pluck the string and see a similar effect to the vibrating tines of the tuning fork, depending on whether you hold it vertically or horizontally.

Behind the Scenes

A computer monitor screen (or a television) looks as though it is illuminated by a steady light, but it's not. A tiny beam of light scans back and forth across the screen from top to bottom sixty times a second. Every point on the screen is illuminated sixty times each second. In between blinks the screen is dark but your eyes see this as a continuous light because they aren't sensitive enough to see such rapid flickering.

A rapidly flickering light is called a strobe. A strobe appears to slow down moving objects. When something moves in front of a strobe, the on-off flashes capture the object in multiple positions, and a sequence of separate positions now appears instead of continuous motion.

A monitor is a complicated kind of strobe. Because of the scanning nature of the monitor's light, an additional illusion is created. This special effect is only visible when the tuning fork is held vertically and is vibrating faster than 60 times a second. It is especially visible when the vibrating frequency (measured in hertz) of the tuning fork is close to some whole-number multiple of 60. Then the solid metal of the tines appears to bend in a wave formation -- and the waves appear to travel down the tines as the scanner moves from top to bottom of the screen. The multiple images of the vibrating tines are in different positions as the scanner moves down the screen. This causes the wave effect. When the vibration is horizontal, the strobe effect simply slows down the motion. Scientists use strobes to measure the frequency of fast moving objects like the timing of pistons firing in an engine.

Article By Vicki Cobb
Education World®
Copyright © 2006 Education World

02/02/2006