A DIY Laser Set-Up to Accurately Measure the Speed of Light

One of the earliest apparatuses designed to measure the speed of light on Earth just got an update with lasers and webcams.

19th century physicist Leon Foucault's low-tech approach to measuring the speed of light involved a candle, set at an angle to a steadily rotating mirror, and a stationary mirror placed some distance away. The lights reflected off the rotating mirror, and then the stationary mirror, and then back to the rotating mirror, which would have moved a set amount by that time. The speed of light—well, technically, the time it took for light to cross the distance between the mirrors—could be easily calculated by looking at the angles of the reflections on the rotating mirror.

This simple and instructive set-up is still used by teens across the world in school, although they usually use lasers, not a candle. You can even buy the hardware to do the experiment online. As you can imagine, these designs are somewhat inaccurate and imprecise because they use outdated measurement tools like microscope rulers, meaning they can usually only get to within five percent of the accepted value for the speed of light.

That's why Gregor Weihs and Zoltán Vörös, professors at the University of Innsbruck in Austria, gave the system a DIY makeover with a small laser, a spinning mirror scavenged from a faulty printer, a webcam to capture the reflected beam, and data analysis tools to calculate the end result.

"Among other things it teaches [students] how to use image processing to obtain data," Weihs told me in an email. "Because we achieve a relatively high accuracy we encourage them to think about the remaining systematic errors—things that don't work the way they should according to theory. This is a very important skill, checking one's assumptions and trying to figure out what else could be wrong with certain simplified pictures."

This new set-up's accuracy is due to several factors: the high precision of the laser used, close monitoring of the mirror's rotation using light sensing diodes—the mirror's speed was also digitally controlled—and the addition of a beam splitter that redirected the laser to a webcam, which captured the resulting beam. The data was analyzed by students using specialized software. The resulting measurements fell within one percent of the accepted value for the speed of light; a huge improvement over earlier designs.

While the resulting device isn't exactly a huge step forward for science, it's sure to teach students a few things about working with data, lasers, and physics. And, Weihs told me, "It was fun."