You and Your Students!
Vicki Cobb, Education World Science Editor
Make a tuning fork appear to wiggle weirdly.
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.
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.
Create a blank screen on the monitor by opening a new file. (If you use a TV screen, turn the dial to "video".)
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.
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
Copyright © 2006 Education World