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97,476 Views ⢠Nov 30, 2023 ⢠Click to toggle off description
3Blue1Brown's video:    â˘Â But why would light "slow down"? | Op... Â
Part 2:    â˘Â I don't know why light slows down in ... Â
Experiment:
If youâd like to try this experiment at home then youâll need a phone with Lidar or you can get a laser meter quite cheaply. The app I found most reliable for the measurement was this one: apps.apple.com/us/app/lidar-measuring/id1535032210
Code:
The 3D simulation is here: github.com/mithuna-y/speed_of_light_in_a_medium/trâŚ
References:
The Feynman lectures- âCh 31: On the origin of refractive indexâ and âCh 48: Beatsâ
Matter and Interactions 3rd Edition - The quote is from section 24-4 on page 1001. Itâs a long passage but I tried to paraphrase it accurately. Hereâs the full quote:
How might we measure the speed of propagation of an electromagnetic wave?
One can think of two different approaches:
(a) Follow a wave crest: If you watch one particular wave crest, you will see
that it travels a distance Îť (one wavelength) in a time T (one period).
Therefore the speed of the crest is
v = Îť / T
(b) Time the arrival of a radiative electric field: One could imagine a different way of measuring the speed of an electromagnetic wave. Suppose that you and a friend synchronize your clocks, then travel to locations that are a distance d apart. Your friend aims a laser at your location, and precisely at time t1, turns on the laser. You record the time t2 at which you first detect the radiative electric field. In the laser light, and knowing the distance between the locations and the elapsed time Ît = t2 - t1, you calculate the speed at which the laser light traveled toward you:
v = d / Ît
In a vacuum, these two ways of measuring the speed of a sinusoidal electromagnetic wave will give the same answer: 3 x 10^8 m/s. However, this will not necessarily be the case if part or all of the space through which the light wave travels is filled with a medium such as water, glass, or even air. In this case, method 2 (measuring the time required for information about a change in the electromagnetic field to travel a given distance) will still give 3 x 10^8 m/s. However, method 1 (timing the interval between crests in a steady state electromagnetic wave inside the medium) will give a different answer, which will almost always be less than 3 x 10^8 m/s.
Views : 97,476
Genre: Science & Technology
Date of upload: Nov 30, 2023 ^^
Rating : 4.931 (65/3,682 LTDR)
RYD date created : 2024-05-21T20:26:15.026302Z
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YouTube Comments - 621 Comments
Top Comments of this video!! :3
I rarely comment on videos, but for this one, I feel I must. As someone who teaches physics and continues to study it in my free time, I feel like your video is such an honest and amazing look into the scientific method. You found something you were curious about, did research, came up with a very well argued hypothesis, tested your hypothesis experimentally, and found the result that you didn't expect, which you finally accepted. You should be really proud - I'm definitely going to be showing this video to my students every year.
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"Light's whole purpose is to harass charged particles." That means that playing with a laser pointer and a cat is fundamentally correct: a cat is just a really really BIG charged particle.
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I remember measuring the speed of light in my college lab 52 years ago. I am so blown away with how a person today can challenge physical concepts with commonly available devices.
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As an researcher with a experiment-observation job it pleases me to see a theoretician struggle with experimental problems - this is not just a little embarrassing but a lot embarrassing to admit. But the most important thing to remember: It is when we find out that we're wrong about something we have the opportunity to learn something new! Great job!
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Some high frequency traders made a lot of money by building a radio connection between stock exchanges, since it was faster than the existing fiber optics connection (speed of light reduced to 2/3 compared to air).
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I've been following you for about the last ten years, and although you don't post often, your videos are of utmost quality, and I truuuuuuly love them! So glad to have this tonight, plus it's a question that bugged me for years so I look forward to your discoveries and explanations!
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The "original" wave does indeed travel at c through the medium. However, what comes out at the other side is NOT the original wave but a superposition of it with the (non-resonantly) stimulated dipole emitters in the medium. Every infinitesimal "layer" of the medium adds a phase delay to the emitted wave (because of the molecular harmonic potential, but that's not important at this point), and this delayed wave interferes with the original wave and shifts its phase by a little bit - but the resulting wave still travels at c. The thing to realize is that this tiny phase delay happens continuously at every single layer of the medium, so you're continuously adding a bit of delay to the original wave at some constant rate which is the same as changing the (spatial) frequency. That explains the change in wavelength inside a medium in the 3b1b video.
In your case, you're dealing with short pulses of light and the TOF LIDAR measures when the peak of the pulse arrives back at the detector. This peak is slowed down inside the medium due to the same superposition mechanism with continuously phase-delayed versions of itself so it takes longer to pass through the medium. That's the definition of group velocity. What stays constant is the phase velocity which is the oscillation speed of the E-field underneath the pulse envelope. In a simulation you can see that the wave peaks underneath the envelope travel faster (i.e., at c) than the envelope itself (at c/n), but because of the interference you never detect these c-speed wave peaks at the output after the medium because they are destructively suppressed by the pulse envelope effectively "riding the wave backwards" (from the reference frame of the input wave). There's also broadening of the pulse due to group velocity dispersion in most media, but that's a story for another day...
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just wow. the journey was crazy, and we've got a mouthful of emotions - somewhat of a recurrent theme of scientific research (and of your channel) is the expectations, the joy in anticipation of the results, the moment reality brings you back to earth with the experimental results. and we've got all of that filmed. incredible.
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So good to see someone who can admit when they're wrong. You shouldn't feel bad about being wrong, it's a way to learn.
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Light is always an electromagnetic wave. It is actually very simple.
You may have missed that the electron keeps accelerating for the full positive electrical field of the wave.
So their generated dielectric wave is always 90degrees (1/2 PI) in phase off (is that + or - 90 degrees?).
The dielectric wave slowly replaces the original wave, per atom-layer.
So after a small layer the original light is no more.
That is why the light "slows down".
There are also fun relativity experiments with light through moving water.
The more difficult part is how the electrons stay in the electronshell.
The electron stays in the shell, even when it is harassed by alternating electric fields (light).
Except when the frequency is matching the electron-shell resonance.
That is where "quantum magic" happens.
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I just wanted to take a minute to say I absolutely love how this was presented. A personal journey of discovery that I felt like I was making with you. I'm glad you didn't shy away from showing your failures as it really helped me to understand the topic much better. Well done!
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The speed of light being slower in fiber optics is pretty important to understanding network latency, I assume similar principles apply there.
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29:20 "You can't make this stuff up"
ChatGPT: Hold my beer.
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I did a PhD in physics many years ago and thats a really excellent video you made. You demonstrate the scientific method, repeatedly testing theory against experiment (reality testing), revising your theories, whilst recognising the complexity of experiments that are not as simple as they might seem. The scientific method is so important because its the only way to bootstrap new knowledge (avoiding theorising in the absence of experiment, which is very tempting as it avoids the hard work of experiment, which is philosophy).
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I like the way you made this video. Taking us through the whole process rather than editing it to show just the outcome is very genuine and I can appreciate that.
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I can't get get over how cute those anthropomorphized electrons are.
Excellent pair of videos, here. As usual. I haven't seen you in my recommended feed for quite a while so I'm very glad Grant pointed me in this direction!
9 |
Great video!
The reason your phone doesn't detect touch-screen input while underwater, is because the screen works by detecting a tiny electric current from your finger ('capacitive touch screen'), and since water is conductive, your phone is basically detecting that "the entire screen is being touched everywhere that the wet plastic bag is touching it, all at once".
It's the same reason why my phone's Fingerprint Sensor doesn't work when my hands are wet, and it actually knows well enough to say 'dry your finger and try again'.
You could potentially use some object, maybe two pieces of foam or rolled up paper towel, to keep the rest of the bag from touching the screen until your finger presses down on it? You only need enough 'functionality' to double-tap
But you've already found an answer, so carry on!
(...I think it's funny how I didn't understand a single thing until you explained why that one formula was 'potentially missing a set of brackets', and the rest made sense to me from there. I am not normally mathematically inclined.)
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I really loved this whole approach of thinking critically about the popular explanations, consulting multiple good-quality sources, putting your own understanding to the test, and even sharing when your experiment shows your understanding to be wrong. An excellent and honest demonstration of how science is supposed to work.
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this is some seriously inspiring science, it takes a lot of courage to post something like this! Looking forward to the next video!
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@LookingGlassUniverse
5 months ago
Part 2 of this story: https://youtu.be/uo3ds0FVpXs?si=TaUPxDsOUi2k4yW5 3Blue1Brown's explanation of Feynman's proof is so great: https://www.youtube.com/watch?v=KTzGBJPuJwM Here's a doc with more of the mathematics behind all of this: colab.research.google.com/drive/1L9X_tq-Kjt-foEhcn⌠The 3D simulation lives on my github. I would love to update it though, so if you have ideas or time to work on it, I'd love to hear about it : github.com/mithuna-y/speed_of_light_in_a_medium/trâŚ
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