by Chris Woodford. Last updated: September 13, 2022.
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 brieflyon the wagon wheels: the wagon is moving forward but the wheels,inexplicably, are slowly turning backward! You stop thinking about cowboysand the pioneer spirit of the west and start reflecting on science.How can wheels moving forward appear to be turning in the oppositedirection? 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 fromphotographic flash lamps to police sirens and warning lights for deafpeople. 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 ofUS Marine Corps and Wikimedia Commons.
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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 thishappening in real life, only in movies—and there's your clue as towhat causes it.
Movie cameras (the forerunners of modern, video camcorders)make moving pictures by taking roughly 24still photographs (known as frames) per second.If there are 24 frames being taken eachsecond, 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 themovie camera snaps a photo. In the 1/24 second it takes until itsnaps the next frame, the wagon wheel rotates so that each spoke hasturned on exactly 1/24 of a complete rotation. In other words, eachspoke 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 eachspoke will have moved on—but not quite enough to catch up with the position occupied bythe 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 sloweddown) 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 reallife too. Make yourself a large disc out of cardboard or hardboard and cut evenlyspaced, radial slits into it (ones running from the center toward thecircumference). Set the wheel spinning (either with your hand or,better still, with an electric motor), look through the disc at amoving wagon wheel (or anything else) and your eyes will get repeated"snapshots," much like the frames taken by a movie camera. Aninstrument 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: thezoetrope,phenakistoscope,and praxinoscope.
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. Ifso, 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 astroboscope. Imagine you're looking at a wagon wheel trundling downyour street, only at midnight. It's pitch black so you can'treally see the wheel, much less those pesky spinning spokes. Suppose you flick on your flashlightvery 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, andthe wheel was rotating at the same speed as before, the spokes wouldflicker 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.
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 thisquickly is virtually impossible. Ordinary lamps work by a processcalled incandescence, where electricity flowing through a filament(thin coil of wire) generates heat andlight at the same time.Incandescent lamps may appear to come on the minute you flick aswitch, 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 evenlonger to work, so they're no good either. What we need is a lampthat makes a bright, instant flash a bit like a mini bolt oflightning—something like the xenon flash lamp in a camera. Now in acamera, flash lamps often take many seconds to activate, becausethey're powered (through a capacitor)by low-voltage batteries.With a high-voltage power supply, rapid charging isn't a problem—andxenon lamps like this can be made to flash on and off dozens of timeseach second. You can also make a strobe flashlight byputting a stroboscope—a rotating disc with slits cut into it—infront 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.
Artwork: How an electromechanical strobe light works. This unit is designed to measure thespeed 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 wheelperiodically 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 offso many times a second. I'm not going to go into details here about how timing circuits work, but youcan find a few specific examples in the Further Reading section below, in the Patents section.
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:
- On the serious side, they're widely used in industry to studyhigh-speed machinery. Look at a rapidly rotating engine under astrobe light and you can see its moving parts as though they werecompletely still (so you have a way to inspect a machine withoutactually turning it off).
- If you need a machine or an engine to spinat a precise speed, you can use a strobe light flashingat the same speed to check: when the speed is correct, the strobelight should make it appear motionless. Old-style record players often have littlereflective dots around the edge and a small strobe light on the side;if the turntable speed is exactly 33rpm (the correct speed), thedots appear to be stationary when the light is switched on.
Photo: Checking the speed of a record player turntable with a strobe light (red) fired up through a prism. Photo by Wyczochrany courtesy of Flickr andWikimedia Commons published under a Creative Commons (CC BY 2.0) licence.
- Strobe lights are also used in medicine, for studying people's vibratingvocal chords. Recent research has raised the possibility ofstrobe treatments for dementia, but it's still at a very earlystage and big questions remain.
- Lights on emergency vehicles (such as squad cars) are sometimesbasic rotating lights inside colored plastic, but strobe lights arealso used for this purpose because they're brighter and more attention-grabbing.
- Telephones and doorbells forhearing-impaired people often have an attention-grabbing strobe lighton them in place of (or as well as) an audible ringer.
Photo: Scuba divers use strobe lights to attractattention underwater in emergencies. Photo by Kenneth Abbate courtesy of US Navy andDVIDS.
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On this website
- Light
- Xenon lamps
On other sites
Activities
- Build your own stroboscope by Nicole Catrett and Walter Kitundu. A simple DIY project from MAKE Magazine.
- PIC Stroboscope by blinkyblinky. How to make your own party strobe, from Instructables.com.
Articles
News
- A Tweet to Kurt Eichenwald, a Strobe and a Seizure. Now, an Arrest by Cecilia Kang. The New York Times, March 17, 2017. Hostile tweets containing strobe images could amount to criminal acts.
- Strobe lighting provides a flicker of hope in the fight against Alzheimer's by Hannah Devlin. The Guardian, December 7, 2016. Strobe lighting makes epileptics ill, but it might just provide the power to help people with dementia.
- Do Fluorescent Lights Trigger Migraines? Tara Parker-Pope. The New York Times, September 2, 2010. Is there any evidence that flickering strip lights can cause headaches?
- The Man Who Stopped Time and Opened Worlds by William J. Broad. The New York Times, January 18, 2000. Remembering Dr Harold Edgerton, pioneer of flash and strobe photography.
From the archives
- Stroboscope stops motion of spinning machinery by Irvin Walters, Popular Science, November 1944. An old but still very interesting article from Pop Sci.
- Whirling wheels stand still when viewed through homemade stroboscope by Gaylord Johnson, Popular Science, August 1936. Another great, classic article!
Patents
Although these aren't helpful to beginners, they offer much more technical detail on how stroboscopes actually work. Some include detailed circuit diagrams:
- US Patent: 4,117,395: Digital stroboscope-tachometer by Robert H. Redfield Power Instruments, September 26, 1978. A high-intensity, precision strobe lamp for measuring the speed of rotating machines.
- US Patent: 2,731,577: Stroboscope lamp by Oscar H. Floyd, Kemlite, January 17, 1956. This goes into some detail about the design of an actual strobe lamp (rather than the timing circuitry).
- US Patent: 1,909,103: Precision frequency system by Emil H. Greibach, Westinghouse, May 16, 1933. An essentially mechanical stroboscope driven by an electric motor.
- US Patent: 1,858,985: Stroboscopic apparatus and method by Peter Davey, Vibroscope, May 17, 1932. An electro-mechanical stroboscope made from a neon lamp, transformer, and semi-mechanical "interrupter" unit that controls the strobe timing.
Videos
WARNING! These videos contain flashing, strobe-light effects.
- Strobe Light Animation by Jennifer Morrow. This neat one-minute video nicely illustrates how a strobe light works.
- Freezing a fan with stroboscope by OrmuScout. Another neat demonstration of the stroboscopic principle.