A string of LED Christmas lights, a circuit tester with an LED on it, or a neon glow lamp from your local hardware store (available as a button-type nightlight; any nightlight labeled “1/4 watt” will probably have a neon glow lamp in it)
None needed. Just plug in your light(s) and observe from a few feet away. (If the bulbs are kind of big, though—as some holiday bulbs are—you may have to stand several feet away. Experiment to find the best location.)
To Do and Notice
Plug in and turn on your light source, observe it from 3–10 feet (1–3 meters) away, and give it the “Bronx cheer.” (A Bronx cheer, also known as a “raspberry,” is a rude noise made by sticking your tongue out a bit and blowing air through your lips in a way that makes them vibrate.) Notice that the lights seem to wiggle back and forth and flicker.
Try shaking your head rapidly and notice whether the lights still flicker. See if you can find other body motions that make the lights flicker. Try the Bronx cheer on other light sources, such as incandescent lightbulbs and car taillights. Notice whether the light flickers.
What’s Going On?
When you give the Bronx cheer, your whole body is vibrating, including your eyes. You can feel this vibration by putting your hand on your head as you blow.
LEDs flash on and off 60 times a second, and neon glow tubes go on and off 120 times a second. This flashing is so fast that your eyes normally can’t separate the “blinks.” But when your body is vibrating, your eyes are in a different position each time the bulb flashes. As the image of the bulb traces a path across your eyes, it looks like the bulb is moving and flickering. An incandescent bulb won’t flicker when you give it the Bronx cheer because the bulb doesn’t flash on and off. Incandescent bulbs give a steady glow.
Plug a commercial neon nightlight into an extension cord. Tape it firmly in place. Twirl the light in front of you. (Be careful not to let it hit anything.) Notice that you can see the light flashing. Because the light is moving, it’s in a new position each time it flashes: the light traces a path across your eye, and its flashes spread out and become visible.
Find an oscilloscope and set it up so that the beam goes straight across the middle of the screen in about 1/100 of a second. Ask a couple of friends to stand back a few yards from the scope. Tell them that the oscilloscope is an eating detector. With your friends both watching the scope at the same time, ask just one to eat a peanut. The person eating the peanut will see the beam jump up and down. Eating causes vibrations of your skull, including your eyes. If your eyes are moving, the dot of light scanning across the oscilloscope shines on different parts of your eyes and appears to jump around.