by Chris Woodford. Last updated: November 16, 2015.
You're happily engrossed in an old western on TV. All of a sudden, someone climbs into a horse-driven wagon, shakes the reins, and gallops off. The camera hovers briefly on the wagon wheels: the wagon is moving forward but the wheels, inexplicably, are slowly turning backward! You stop thinking about cowboys and the pioneer spirit of the west and start reflecting on science. How can wheels moving forward appear to be turning in the opposite direction? It's all to do with what's called the stroboscopic effect (or strobe effect for short). It's put to good use in everything from photographic flash lamps to police sirens and warning lights for deaf people. Let's take a closer look!
Photo: How do flashing lights on patrol cars work? Some rotate. Others are strobe lights that flash on and off electronically. Photo by Leon M. Branchaud courtesy of U.S. Marine Corps.
What is the stroboscopic effect?
Photo: Why do the wheels seem to turn backward when the cart goes forward? It's all to do with stroboscopes!
First, let's clear up the mystery of the wagon wheels turning backward. You never see this happening in real life, only in movies—and there's your clue as to what causes it.
Movie cameras (the forerunners of modern, video camcorders) make moving pictures by taking roughly 24 still photographs (known as frames) per second. If there are 24 frames being taken each second, each frame lasts one twenty-fourth of a second (1/24s) and there's a short gap between each frame and the next when the camera is not filming.
Imagine if the wagon wheel has 24 spokes on it and is also, coincidentally, making one complete rotation each second. Suppose the movie camera snaps a photo. In the 1/24 second it takes until it snaps the next frame, the wagon wheel rotates so that each spoke has turned on exactly 1/24 of a complete rotation. In other words, each spoke is now at the point where the previous spoke was 1/24s ago. All the spokes look identical, so when the camera next takes a photo, it's as though the spokes were all in the same place as they were in the last photo. Although the wheel is rotating, from the camera's point of view it looks motionless!
If the wheel is rotating just a fraction slower, every time the camera takes a photo each spoke will have moved on—but not quite enough to catch up with the position occupied by the previous spoke 1/24s ago. And that's why the wheel looks like it's going backward. It's a simple example of the stroboscopic effect: the way in which moving objects appear to be still (or slowed down) when we view them under the right conditions (with a stroboscope or a strobe light).
Animation: Suppose this wheel is a rotating cartwheel, with one of the spokes painted red. If you blink fairly quickly, at a constant rate, you'll find you can (with quite a bit of effort), make the red spoke appear to be rotating backwards. That's a simple demonstration of the stroboscopic effect.
What is a stroboscope?
You can see the wagon-wheel effect in movies, but there's a way to see it in real life too. Make yourself a large disc out of cardboard or hardboard and cut evenly spaced, radial slits into it (ones running from the center toward the circumference). Set the wheel spinning (either with your hand or, better still, with an electric motor), look through the disc at a moving wagon wheel (or anything else) and your eyes will get repeated "snapshots," much like the frames taken by a movie camera. An instrument like this is called a stroboscope and it's very easy to make. It works in the opposite way to a movie camera (turning movement into a series of still images) and also in the opposite way to those weird-sounding, early animation machines you may have heard of: the zoetrope, phenakistoscope, and praxinoscope.
What is a strobe light?
Cutting slits in great big wheels might be a bit too "19th-century" for your liking. If so, you might prefer another way to achieve stroboscopic effects: using a rapidly flashing lamp called a strobe light. A strobe light works in an exactly equivalent way to a stroboscope. Imagine you're looking at a wagon wheel trundling down your street, only at midnight. It's pitch black so you can't really see the wheel, much less those pesky spinning spokes. Suppose you flick on your flashlight very briefly then flick it off again. The wagon wheel will light up. Now if you could flick your light on and off 24 times a second, and the wheel was rotating at the same speed as before, the spokes would flicker but appear stationary.
Photo: Strobe lights work in a similar way to the xenon flash lamps used in cameras, but are designed to fire faster and much more often.
Sounds good, doesn't it? Unfortunately, switching an ordinary light on and off this quickly is virtually impossible. Ordinary lamps work by a process called incandescence, where electricity flowing through a filament (thin coil of wire) generates heat and light at the same time. Incandescent lamps may appear to come on the minute you flick a switch, but it takes time for the filament to heat up and cool down, so they can't flash rapidly on and off. Fluorescent lamps take even longer to work, so they're no good either. What we need is a lamp that makes a bright, instant flash a bit like a mini bolt of lightning—something like the xenon flash lamp in a camera. Now in a camera, flash lamps often take many seconds to activate, because they're powered (through a capacitor) by low-voltage batteries. With a high-voltage power supply, rapid charging isn't a problem—and xenon lamps like this can be made to flash on and off dozens of times each second. (Note that you can also make a strobe flashlight by putting a stroboscope—a rotating disc with slits cut into it—in front of an ordinary incandescent lamp.)