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Flashing strobe lights on top of a patrol car.

Strobe lights

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 Shannon Yount courtesy of US Marine Corps and Wikimedia Commons.

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Warning: Strobe lights

If you're going to play around with strobe lights, there are a couple of things you should note:

Please be aware that two of the links at the bottom of this page open videos that contain strobe effects. They are clearly labeled and you don't have to click on them if you don't want to!

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What is the stroboscopic effect?

Wooden wheel on a toy farm cart

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 demonstrating the stroboscopic effect

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.

How to make a stroboscope from a plastic or paper plate

Photo: It's easy to make a stroboscope by cutting thin slits into a plastic or paper plate, which you then spin quickly either by hand or with an electric motor. Using a ruler and a protractor, try making 12 slits (one every 30°), and take time to do them thinner and neater than mine (I'm just quickly demonstrating the idea). You'll find step-by-step instructions in the Make article in the references below.

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.

Xenon flash lamp from a digital camera (close-up).

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.

How does a strobe light switch on and off at a precise frequency?

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. 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. Another mechanical approach is to use an electric motor and something like a cam (an asymmetrical, eccentric wheel) to interrupt the contacts to a strobe light at a precisely controlled frequency.

A cam-driven electromechanical stroboscope from US Patent 1,858,985 by Peter Davey.

Artwork: How an electromechanical strobe light works. This unit is designed to measure the speed of rotating machines and it's based on three separate components: a lamp (yellow), a transformer circuit to make the lamp light up (red), and an interrupter unit (blue) to switch the transformer on and off at a certain frequency. The core of the interrupter is a cam (orange) connected to a rotating shaft powered by whatever machine you're measuring. As the cam rotates, it's eccentric wheel periodically pushes apart two electrical contacts (green, labeled 16 and 18), switching off the transformer and lamp, before allowing the contacts to touch again, which switches the lamp back on. That's a simple mechanical way of making a strobe light flash without using any sort of electronic timing circuit. Artwork from US Patent: 1,858,985: Stroboscopic apparatus and method by Peter Davey, Vibroscope, May 17, 1932, courtesy of US Patent and Trademark Office.

Since the beginning of the 20th century, most commercial strobe light units have worked electronically, using a variety of different circuit designs to switch something like a xenon or neon lamp on and off so many times a second. I'm not going to go into details here about how timing circuits work, but you can find a few specific examples in the Further Reading section below, in the Patents section.

NASA strobe light demonstration.

Photo: A simple laboratory demonstration of the stroboscopic effect by NASA. You put a series of changing patterns, printed on a circle of cardboard, on the front of a rotating fan. As the fan spins, the patterns blend into a single moving image. Now switch off the room lights and switch on a strobe light instead (bottom). If you synchronize the strobe speed with the fan speed, you can freeze the pattern. Photo by NASA Glenn courtesy of DVIDS.

What are strobe lights used for?

Strobe lights have all kinds of uses—from serious and scientific to recreational and fun. Here are a few examples:

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From the archives


Although these aren't helpful to beginners, they offer much more technical detail on how stroboscopes actually work. Some include detailed circuit diagrams:


WARNING! These videos contain flashing, strobe-light effects.

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Articles from this website are registered at the US Copyright Office. Copying or otherwise using registered works without permission, removing this or other copyright notices, and/or infringing related rights could make you liable to severe civil or criminal penalties.

Text copyright © Chris Woodford 2009, 2018. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2009/2018) Strobe lights. Retrieved from [Accessed (Insert date here)]


@misc{woodford_strobe, author = "Woodford, Chris", title = "Strobe Lights", publisher = "Explain that Stuff", year = "2009", url = "", urldate = "2022-09-13" }

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