Twinkle Twinkle little light...
A neon glow tube powered night light flickers on 120 times a second. The flash rate is high enough that it does not noticeably flicker. However swing the neon light back-and-forth or around in a circle and it can easily be seen to be turning on-and-off.
Plug the night light into the extension cord and tape it securely in place, so that when it is swung rapidly in a circle it will not break free. Plug in the extension cord.
To Do and Notice
Dim the room lights, swing the neon tube in a circle, notice the arc of light dashes that follow the bulb around.
Notice how many dashes follow the light.
What's Going On?
Each time the light flashes on, it triggers your retina to fire nerve impulses. These nerve firings continue after the bulb has moved. Each firing is separated by 1/120 s. So if you see 6 dashes following the light, these nerve firings keep going for 6 flashes, they keep firing for 6/120 or 1/20 of a second.
Movie images blink on-and-off 72 times a second.
Television images are presented in two parts for 1/60 second each.
Your eye does not detect the flickering of these images because they remain in the same place. If the television or movie screen were to move across your field of view quickly, like the blinking neon light, you would see it flicker.
Old time movies blinked on-and-off 24 times a second. People could see them flicker so the movies became "the flicks."
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22 May 2000