Speed of Light | Chopping Light Beams

The Speed of Light series consist of five parts. Quick access links are here.

Part 1 | Earliest Ideas

Part 2 | The Eclipses of Io

Part 3 | Chopping Light Beams

Part 4 | Done With Mirrors

Part 5 | Michelson and Morley

In this post, the speed of light is successfully measured by timing a chopped light beam between two distant mirrors.

The first measurement of time for light to travel a large distance was announced by Ole Christensen Rømer in 1676. His work involved the travel of light across the diameter of the Earth’s orbit. It was an inferred travel based upon knowledge of celestial motions of Earth and the closest moon of Jupiter named Io. He did not set out to directly measure light beams as they made that journey. Instead, he was measuring something different and made his discovery by accident.

In this post, we see how the speed of light was calculated from direct measurements of a light beam intentionally sent from one place to another and back after reflection from a distant mirror. Armand Fizeau conducted the experiment in 1849.

Who Was Fizeau?

Académie des Sciences

Details of his biography are here. Fizeau was born in Paris. His father was a medical doctor and professor of pathology. It was assumed Armand would continue in the tradition of his father and previous relatives. He enrolled at Collège Stanislas in Paris. There he became a good friend with fellow student Léon Foucault. Both men made important contributions in several areas of science. Armand did not enter the medical profession. He did start attending the Paris Medical School but quit after suffering from headaches and migraines.

In 1839, Fizeau and Foucault attended a lecture by Louis-Jacques Daguerre on the new photographic process. Daguerre exposed a photographic plate to a window using his camera. He talked for 30 minutes before using a variety of chemicals to develop the image. Fizeau and Foucault were impressed. But, they felt the process should be speeded up so human subjects didn’t need to sit still for so long. They were able to reduce the time to 20 seconds.

In 1845, he and Foucault were asked by Arago at the Paris Observatory if they could obtain images through a telescope using the new process of photography. They were very successful and produced the first image of the Sun through a telescope which included groups of sunspots.

Fizeau also proposed in 1848 an explanation for the shift in wavelength of light from a moving star. This was what we now call the Doppler Effect. He was unaware of the previously published work by Christian Doppler in 1842.

Speed of Light Experimental Setup

Arago at the Paris Observatory suggested Fizeau try to measure and calculate the speed of light between two places on Earth. Fizeau chose two high ground locations, his parent’s home in Suresnes and Montmartre on the right bank of Paris. They were 8633 meters apart. He planned to direct a bright light across the distance to a flat mirror so it was reflected back for him to see. That should be simple enough in principle. But, light can cover that round trip of 17266 meters in less than 0.00006 seconds. How could he possibly time something that fast? He decided to use a spinning toothed wheel to chop the beam into segments.

Consider a toothed wheel like this with a flashlight beam pointing at it. The wheel is not turning. The beam and teeth are situated so it passes through the gap. If a mirror was placed in the distance, it could be positioned so a reflection could be sent back through the tooth gap to be seen by the operator. These toothed wheel models are my modifications of one by Google SketchUp user hamdi40.

Suppose the wheel is rotated slightly so a tooth is in the way of the beam and blocks it. That would prevent the beam from reaching a distant mirror.

Fizeau understood that if the toothed wheel was spinning slowly, it would chop the light beam into long segments. The segments would be able to travel from his parent’s home to Montmartre and back.

The challenge was to get the segments chopped at such a high rate so a segment would go to the mirror and back in the time it took for one tooth gap to be followed up by the next gap of the rapidly spinning wheel. He would need to spin at a high rate of speed and have a wheel with many teeth in order for the time between gaps to be sufficiently quick. Remember, the travel time was on the order of 0.00006 seconds.

Here is an illustration from a publication by Project Gutenberg called The Story of the Heavens, by Robert Stawell Ball. This text says the illustration is from Simon Newcomb’s Popular Astronomy in 1878. It is not the actual toothed wheel Fizeau used. But, it does illustrate the concept in a schematic way.

Simon Newcomb – Popular Astronomy 1878

From the Paper by Fizeau

The actual paper published by Fizeau is in French. A fellow blogger, Skulls in the Stars, posted in 2008 some details about Fizeau’s attempt to do this experiment and how it related to principles of relativity. He followed with a post supplying the text both in French and translated English of Fizeau’s original paper. I am grateful that he found the paper and did the translation.

The illustration above by Newcomb shows a black optical device in front of the observer. A bright, narrow beamed light source was directed into the optical device. It contained a partially silvered mirror oriented at 45˚ to the direction of the light beam. Such a half-silvered mirror would allow it to both reflect light and transmit light. The observer could also see through the mirror in the device.

The toothed wheel used by Fizeau had 720 spokes, or teeth. It was rotated by a clockwork made by Froment. During operation, light shined into the optical device, and was diverted by the 45˚ half-silvered mirror toward the toothed wheel. If the wheel spun slowly, the light would pass through a gap between teeth, travel to the distance mirror 8633 meters away, return, and be blocked from visibility by the next approaching tooth of the wheel.

When the wheel was spun faster, the light would traverse to the distant mirror and back in the same time it took for the next gap between teeth to move one 720th of a rotation. The returned light would pass through this gap, through the 45˚ half-silvered mirror, and into the observer’s eye as a bright spot of illumination. Fizeau found the rotation rate of 25.2 rev/s allowed him to see a bright spot of illumination.


Click to embig


Angular speed rate = angular distance divided by time to rotate. Here, angular speed = 25.2 rev/s.

Angular distance turned was 1/720th of a rev. or 0.00139 rev.

25.2 rev/s = 0.00139 rev/time.

Therefore, time to rotate the wheel one tooth gap was 0.00139 rev/25.2 rev/s = 0.0000552 sec

In that time, light traveled 17266 meters to the distant mirror and back.

Speed of light from his values was 17266 meters/0.0000552 sec = 313,000,000 meters/sec.

Today, the accepted value is 299,792,458 meters/sec in a vacuum, very slightly less through air.

Fizeau was only about 4% in error. That is quite a remarkable result.


21 thoughts on “Speed of Light | Chopping Light Beams

    • Repeated trials also gave him consistent results. Experimenters followed later with multi-sided mirrors that rotated rapidly. They, too, got good results.

  1. It takes quite a bit to slow me down in the morning to read science, but I really enjoyed this. Part of my enjoyment is that I understood a good part of this — you explained it well.

    One thing caught my attention, which may or may not have any significance. When I read that there were 720 spokes, I couldn’t figure out why that seemed notable – until I realized 720 is twice 360, the degrees on a compass. Any relationship? (The explanation for the 720 spokes may be here, or in a link, and I just missed it.)

    • You are correct about the 720. In my reading, I didn’t see any discussion as to why he chose 720. The instrument was made by a person named Froment for Fizeau. The teeth in the wheel are discussed in the two links by the other blogger as he tried to get a translated copy of Fizeau’s paper. Even there, it doesn’t explain why 720.

      I’m glad to be able to ‘slow you down’ to read the post. That makes me feel good. 🙂

  2. It always amazes me to learn how scientists come up with experiments to discover answers to questions they have… Me, I can’t even think of the questions! This was a problem for me when I thought I was going to be a scientist.

    • Generating the questions, or the directions to go to carry out something we want to do…those things can be a challenge. I think we eventually get there as we identify the things that drive us and attract us. For you, it seems to come with painting. You get an idea of what to paint. You choose the colors, layout, time of day, subjects, etc. All those things would be difficult for someone not in that field.

  3. Science history is fascinating to me, Jim – great job here. I hope you are planning to continue and cover the Michelson-Morley experiment.

    The nineteenth century was a time of explosive interest in science of all kinds, a fact forcefully presented in an excellent book I just finished, The Remedy: Robert Koch, Arthur Conan Doyle, and the Quest to Cure Tuberculosis – Thomas Goetz. (Public confidence in rational analysis was on the rise and, surprising to me, was significantly popularized by the publication of Dr. A. Conan Doyle’s series on Sherlock Holmes.) Work by thinkers like Fizeau also, no doubt, caught the public’s imagination. This demonstrates once again how science progresses by the process of peer review and reproducibility.

    Like Shoreacres, I too noticed the significance of the 720. It’s purely speculation on my part, but it seems logical to me that the spinning disk would be laid out with a standard protractor which would have been marked in degrees.

    • The M-M experiment will be in a post.

      The interest in science in the 19th century was high. It had to be such exciting times. We’ve seen the great advances continue into the 20th and 21st. The sad part of this story is the growth of public distrust by a large part of society. It is distressing.

      I think the splitting of the toothed wheel into 360, then 720, was a natural thing to do with a protractor. It was probably such a natural thing that they felt no need to explain their rationale.

      Thank for your kind words and your comments.

  4. I’d heard of Fizeau but not the details of his clever experiment. It’s hard to imagine that world of the early 1800s when people understood so little about many physical processes, but then our descendants a few centuries from now will probably feel the same way about us.

    It also occurs to me that I probably shouldn’t refer to “us” in such a general way. I suspect that if you took a random poll, many people today wouldn’t couldn’t come close to giving an estimate for the speed of light, or might not even be aware that light moves at all.

    • Sadly, I think you are right about the ignorance factor. Jay Leno would do his bit asking people on the street about things. Jaw-dropping answers.

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