A curiosity discovered in the 1800s, the stroboscopic effect led to movies and detailed studies of motion. People who watched a rotating disk through another with slots cut near the edge saw the observed disk slow down, stop or reverse. The slotted disk developed into the Victorian zoetrope, and then the movie camera's shutter. Electronic strobes with xenon flash lamps or LEDs are now used to exploit this effect for entertainment, science and industry.
Aliasing
The stroboscopic effect is a visual form of temporal aliasing. This happens when you see motion through a shuttered mechanism, like a movie camera. You don't experience the action continuously; it comes through as a series of interrupted time slices. If the object in front of the shutter isn't moving, or if it's moving in a straight line, it will still appear that way. If the motion is circular or repeating, like a spinning wheel or a boy on a pogo stick, the motion will change. If the shutter and wheel speeds are the same, the wheel will appear stopped. If they're different, the wheel will seem slowed or look like it is going backward.
Stroboscope
A modern stroboscope is an electronic device. It has a bright light that flashes at a controlled rate. It may flash from a few times per second to hundreds or more. In a darkened setting, the strobe creates the shuttered effect by showing you action in glimpses. The strobe may be synchronized to the action, letting you freeze it.
Movies
The "wagon-wheel effect" is a classic example of a stroboscopic effect in the movies. As a horse-drawn wagon passes by the camera's view, the spoked wheels appear to move too slowly, or stop, or rotate backwards. This is caused by the wheel spokes aliasing with the 24 frames-per-second shutter rate of the movie camera. This also affects the motion of fan blades and car wheels. Television screens appearing in movies need special treatment; otherwise, the TV's scan rate will conflict with the camera shutter, giving an erratic image.
Photography
A stroboscope used with a camera can create a number of effects. It can freeze fast motion like flying bullets or bursting balloons. It can create a multiple exposure of motion, like a golf swing, frozen as a series of instances, all in the same image.
Uses
Engineers can use a stroboscope to slow the motion of machinery and see the cause of problems. An athlete can use it to capture and analyze his technique. Night clubs use them to emphasize musical beat and slow the dance movements.
Warning
Stroboscopes can trigger epilepsy in people predisposed to it. Most party strobes are limited to 10 to 12 flashes per second, though scientific models will go faster.
