Measuring the speed of light the old fashioned way: Replicating the Fizeau Apparatus

In 1849, the first terrestrial measurement of the seed of light was made by Hippolyte Fizeau using a bright focused lamp, a spinning slotted wheel, and a reflector a few kilometers away. This famous experiment is one that I've spent the better part of the last year thinking about and very slowly replicating! Using a 5mW green-dot laser sight, a "slotted disk" laser-cut from black posterboard, a speed-controlled dremel, a digital camera, and a full spool of retroreflective tape, I succeeded in measuring and calculating the speed of light! In this video I introduce and explain the theory behind the experiment, and actually perform the experiment to calculate a result. (You'll have to watch to see how close I got!)

Future videos in this series:
-Repeat experiment with 10-mile round trip for the light
-Mechanics of the Fizeau Apparatus (incl. wheel speed control and measurement)
-Optics of the Fizeau Apparatus (incl. retroreflectors)

Music in this video:

[Original Score] by Wyatt W., used with permission

[BluBr]

Adjusting for the refractive index of air (1.000293) you are are actually at 99.4% of the speed of light. I wonder if it is even closer considering that the air is (probably) humid and further slowing down the light. Amazing video 👏👏 I love how you made the speed of light so much more accessible and "real". This is one of the best videos on YouTube

[JayDAnderson]

When you consider the true speed-of-light (actually speed of propagation) at 10M above sea-level (about where you are) and also at about 75% humidity (standard RH near salt water) the refractive index increases due to greater atmospheric density and humidity increasing the propagation time of light (slowing the speed-of-light). So your results may actually be "spot on". Quite amazing.

[MathisWellz]

You took almost a whole month to put this whole thing together. You are amazing, man. Thank you.

[adamrasmussen]

Really makes me appreciate just how much effort it took to 'science' 2000, 500, even 100 years ago. Even today, with all the modern tech toys that exist, this was still (at least) a month long job just to recreate an experiment that you already *know* works..pretty impressive. Just imagining the practical undertaking that this must have been 150 years ago hurts.

[rene]

So, if he was 5% high, and you 1% low, the logical conclusion is that the speed of light slowed down by 6% over those 175 years ;P

[Justducks]

This is a criminally underrated channel. These videos combine the knowledge and practice required for these experiments and demonstrations incredibly well. This channel needs more subscribers.

[tombiffin]

This was awesome, I've never seen this experiment replicated and the speed of light is such a cornerstone of physics. You put so much time and effort in to this and it's really appreciated. Good job.

[Sir_Uncle_Ned]

Bloody hell. Such a simple method produces such an accurate result. Talk about standing on the shoulders of giants!

[DutchPhlogiston]

Wow, great! Original, explained well, and I love how you show all the troubleshooting you had to do.

But most of all, I now have an EXTREME level of respect for Fizeau, who did this more than 150 years ago without lasers, cameras and digital tachometers, and most importantly, without knowing in advance what the result should be (at 13:11 you explain that you got a value of 93% c, but then found the result could be improved by correcting for camera movements. An advantage Fizeau did not have).

[newtonbomb]

I'm glad you've stuck with this channel despite it not getting the traction it deserved all this time. Lots of great content in your library and still making more!

[DinBoots]

“Gravity is 10 and Pi is close enough to 3 that nobody cares.” *Engineers wince*

[futureshock]

Fizeau is an absolute beast, managing to do all this 170 years ago
It really is crazy how old school scientists did all their work with none of the modern tools we take for granted. Also good job recreating this, ive heard about or vaguely seen the diagram of spinny wheel as a kid but never actually learnt how it works or seen it in action. Thanks for making this vid it really does put a smile on my face.
Also "π is close enough to 3 that nobody cares" I dont know how I feel about this statement having played the drawing internal and external polygon game when i was younger to find pi manually

[olivialambert]

From one physics graduate to another, amazing. I wish I had thought to copy experiments when I had the chance. This has a lot of the intrigue and excitement of copying the experiment myself, but with none of the energy investment. I've a few friends who ended up teaching, this type of video seems perfect to build interest in the type of person considering a physics undergraduate.

[rafaelreusch]

It's incredible that we can have this level of content and knowledge here in youtube.

[WhatsACreel]

Such a great video!! Experiment, presentation, editing, all of it! You nailed it brus! Have a good day mate.

[simaomj]

This makes me want to go outside and perform the experiment myself! It's reassuring to know that the value you obtained is so close to the accepted value of c. Doing a proper experiment is a laborious task, and I'm happy you took the time to produce this video and put it on youtube so that we can go on this journey with you. Thank-you!

[kylegreen]

The quality of your content continues to impress.

[jomaloro]

Wow, just wow.

Hooe you get big here in Youtube someday.

[trevorkohan]

This dude is incredible. His passion is contagious and his excitement as he’s describing things keeps me engaged. I wish I had professors like him in Uni

[r.i.p.volodya]

VERY well done!!! You clearly have what it takes to be an actual experimental physicist. Bravo!