Why does light slow down in water?
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- čas přidán 26. 04. 2024
- There are many mysteries of physics for which you can find explanations online and some of those explanations are wrong. In this video, Fermilab’s Dr. Don Lincoln takes on the mystery of why light travels slower in water and glass. He lists a few wrong explanations and then shows you the real reason this happens.
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It took me 55 years and 10 minutes to understand. Thank you!
And 24 seconds
Hahaha good one!
Happy birthday
@@nikhilnegi9446 good one !
How old are you now
Anyone else coming back to this video every few months because you forgot how it happens and you're thinking about it again?
yup! that's me-.-.-.but it's my 2nd visit-.-.
My second visit 🤠
Third visit
It's not a surprise. Nonsense is easy to forgot. Since this explanation is not just factually wrong, but also can not be quantified, nor to give a qualitive explanation of any experience, it fails to build into working cognitive pattern. How could 2 waves be superimposed if the slower one is not present near the wavefront?
The origin of the badly presented theory is this:
en.wikipedia.org/wiki/Ewald%E2%80%93Oseen_extinction_theorem
It is similar, but there are some elementary differences: the wave emitted by the electrons has the *same* speed as the original light, this is how it can cancel the original wave at the wavefront. This is some of the explaining power of this model: the generated EM waves travels with the same c speed as the incident wave. This power is missing from Don's false explanation.
Yup
Glad you recorded: Why does light bend when it enters glass. Recommend people view that one first, to assist with understanding this video.
I was just asking myself why it bends.
Another easy to follow video. The more we study and watch these videos, the more knowledgeable we become. Thank you Dr Don.
Knowledgeability is like the money in your wallet. Your decisions what to buy, and when, and how, is the giant rest of the universe.
Too bad it can't be true. Light travelling through a straight tunnel would have its speed altered by filling the tunnel with microwaves. This is not the case. Also it would affect light travelling through space. Over vast distances, the small amount of radiation in the cosmos would cause light to get blurred, which does not happen.
too many easy to follow videos, they all rephrase the observation instead of answering the question
@@governmentis-watching3303 You're wrong.Microwaves don't change the speed of light because they also travel at the speed of light, like any electromangetic wave.
Excuse me while I pick my jaw up off the floor.
It's fun seeing your comments in the videos I watch.
I know right, I figured it was the scattering explanation before I watched the video.
Uh dunno. When waves interfere, their amplitudes add up, not their speed. And afterwards the merged waves do not unmerge.
Oh c'mon, do you subscribe all the channels that I subscribe ??
Ah! Try to subscribe your own channel like I do! Got you!
Wait, you can actually subscribe the Blab with the Lab and vice-versa...noooooooo! :)
@@Geo_Knows_Things the waves do not merge "physically"
We perceive them as a single wave when they're superposed. Really they can be seen as any number of waves , you can look up the Fourier transform if you're interested. The universe doesn't care, as long as the electric field is following the laws, any combination of waves could exist. What matters is the overall effect of these waves. If it's the same for 1 wave or the sum of 2 different ones , then either way is fine.
Once light exits the glass, the second wave is no longer generated (since it only exists inside the matter where electron are) and the "resulting" wave is the same as the starting wave.
As for the speed, I have multiple answers conflicting in my head, I'm not sure yet.
You said only the amplitudes are added, technically yes, but the speed and other characteristics are just a way to describe the change of amplitude.
The amplitude is not constant, both in time and space, so adding the amplitudes of the 2 waves in each and every point in time and at each and every point they occupy in space will inevitably alter the perceived speed/phase/frequency of the resulting wave.
I'm an electronics engineer and we study Maxwell's and wave equations, of course. Optics is taught in high school. I've never really stopped to think how these two relate and your video made it crystal clear. Very interesting!
I remember that in university on a wave physics course we where taught that η1/η2==ε1/ε2. It made sense back then, but I had not a good explanation until this video.
@@jorgealzate4124 The second wave is fiction, so this video adds more confusion.
@@dreamdiction Of course, there is no second wave in the sense of a "stationary wave" but there will be a second inducted wave as long a electromagnetic wave travels through the material. Thats because the way a electromagnetic wave propagates, it induces an electric (and a magnetic) field in the propagation medium (even in the vacuum), and because in this case, the fields are moving, so they are the induced field(s), which in turn will interact with the incoming wave, and all of this results on a localized, altered wave (please physists don't kill me, I'm tryng to make myself clear, im just an engineer doing office work :'( ), then the traveling wave properties will be related to the electric permitivity of the two mediums.
@@jorgealzate4124 as an engineer you should know that mixing of waves gives you sum and difference frequency waves. However this is not the case in the medium, as by fundamental law the frequency stays the same, only the wavelength changes. Collosal error in this video.
This is what happens when you opt stem but you can't understand it 🤣🤣🤣
Fantastic video! There's one key piece still missing for me: "light still moves at the speed of light, but the waves from the electrons move at a different speed, and the combined wave moves slower than light would..." My question is: how can the waves from the electrons move at a different speed? All electro-magnetic radiation moves at the speed of light... how can the waves from the electrons be slower? They'll be moving in a different direction, but not slower. No?
Exactly, he functionally didn’t explain anything
@@igorjurdana1311 because he is wrong, it doesn't work like that, the speed is the same, it is the PHASE that goes back slightly, really the speed of light is the same, it is the appearance that the phase gives that makes us believe that it is going slower
Always joy to watch your videos Dr. Lincoln - even If its way over my head! Excellent explanation! However, given the wave particle duality of the electron, I feel like there ought to be an explanation that uses the particle view as well...looking forward to that video :)
I think that everything is vibration: a wave in a field. WE just perceive things as particles, thus, when measured, AS a particle, we get particle-like answers. However, every physicist, and most people, understand that there isn't ANY matter. Thus, no matter to be broken down into particles.
You may have a long wait. Or just ask God, now.
5:32 "...when an atom has been absorbed by a photon..."
Did I miss something?
Electrons absorb photons when they subjected to them. They re emit it after some time
@@mubashir22ful I know that. But he said "when atom has been absorbed by a photon" which is the other way round😂
Yes you did. You missed everything. Atoms are not absorbed by photons, also this is a wave phenomenon, not a particle phenomenon.
Wayne Adams Yea I think it was kinda the joke that photons DONT usually absorb atoms.
PastafarianBEAVIS PROscience Wow that was a very scientific explanation for saying the wrong words, lets hope it really was just that...
Damn one of the most straight forward explenations i've seen on youtube, while also explaining common misconsceptions: brilliant, definitly a sub.
The biggest misconception about light is that physicists think that photons are point objects where time and space don't exist, including Fermilab. Well truth is Photons have a volume proportional to the their wavelength. Photons definitely experience time and space as well. Einstein is wrong.
@@johnnybinghamton2117
Einstein was wrong and Johnny Binghamton is right. What a surprise... Have you written an article to back up your claims?
@@johnnybinghamton2117 here, take your Nobel 🏅
@@johnnybinghamton2117 why dont you write down equations that prove your point, and lets have observations and experiments that support your claims and equations. like Einstein did. until then, cease and desist
@@johnnybinghamton2117 If the electrons moves, that means the light looses energy and wouldn't regain speed in the same direction. I think the best explanation is that light doesn't interacts with the glass at all. The glass expands space-time, the more space, time and speed slows down, it's the relationship between space and time from relativity theory.
Something I found interesting is a physics lab I had in school where we measured the index of refraction of opaque objects. Up until that time I thought only transparent object had an index of refraction.
It's indeed fascinating, light and x-rays are both just different wavelengths of the electromagnetic spectrum. Different materials are more or less transparent to different wavelengths. So what appears opaque in one wavelength is transparent in another. When working with materials/shaders in 3D programs, IOR is also important for opaque objects as it controls their reflectiveness. There is also a distinction between dielectrics = wood, glass, plastics (strongly bound electrons) vs conductors = metals (freely flowing electrons). :)
@@SuperemeRed I felt like a genius when reading Project Hail Mary by Andy Weir and the alien left a bunch of opaque tiles except one - I realized imediatly because this fact about the EM spectrum stuck with me. Really great book. If you have not read it, I recommend it.
Thank you sir. I have a question at 9:19, “The wave from the electrons move at a different speed”, why it is different?
I think it's (at least partially) because electrons have a tiny amount of mass which prevents them from moving at the speed of light. Photons don't have mass which allows them to move at the speed of light
@@myagrimm4719 great explanation
Please Bro if you find the answer to your question can you explain to me why the wave created by the electrons moves at a different speed 9:20 and also why the combined new way moves slower than the speed of light 9:43 ? Please bro I didn't find why...
They don't. The video fails to explain why that it.
@@Khalid-Ibn-Al-Walid
The combined new wave does travel at the speed of light. The thing that is sower is the group velocity, which is what we are interested in.
But why does it bend at an angle after hitting the glass?
Sunny shah Light always takes the shortest path...
I think it‘s because the lightwave gets influenced by the electrons. When it has passed the material, it gets faster again and changes the direction back to its original
.
This can be explained with Huygens principle of waves. It states that every point (particle if the wave is mechanical, i.e sound) touched by a wave starts propagating it by its own.
Because of, you know, singular points propagating the wave, these propagate it on a sphetical way, not straight. But they end up combining to build the shape you put in the water without scattering. This is why you can see the tip of the pencil submerged under the water, not a giant yellow blob of light.
*i didnt really know how to explain this part, so please search "huygens principle" on google images and use any of the first results to ilustrate my explanation. Or look for a better one while you are at it, whatever*
Lets imagine you can stop time just on the moment the light of a laser is going to touch the water. Because the laser is kinda tipped, the light comes at an angle, and the light "front" is perpendicular to the ights direction, so parts of it will touch the water before others. When the light REALLY STARTS to touch the water only one of the tips of the "front" _is_ , the rest is still just hovering on the air. The particles of water touched by the tip propagate the laser light, but a big chunk of the "fronts light" emitted by the laser isnt touching the water yet. By the time another chunk of laser has touched the water and the points have started propagating it, the "sphere of light" propagated by the particles touched before is bigger. When the laser has entered the water entirely, the spheres of light propagated by the points touched first is significally bigger than the spheres of light coming from the points just touched. Then you can connect the spheres with a straight line, that shows you the new wave "front". Knowing that the "front" is perpendicular to the wave, then you can draw the new wave direction, and see *not only does it change, it is **_more vertical_* .
I hope you understood this or helped you and the lurkers looking for answers to this xd.
Light must synchronize with the shape of a structure, resulting in deflecting and vectoring do to the principle of two or more elements attract called Gravity, light must randevu around elements like objects do among solar system and planets in the path, it must like so with nutrons,atoms ions etc..., because it cannot actually go through an element or it will no longer be, so until a path is merged by structure form.
Thanks, Dr. Don Lincoln. I've taught refraction for years. I'm talking pre-internet. But today, via your video, I really got to the heart of the reason. I totally dig your engaging and warm style and also your super cool physics wardrobe!
Oscillating electromagnetic fields with photons of light and electrons of matter having most of the fun (with nuclear weapons and radiation occasionally doing heir things with strong nuclear force and weak nuclear force respectively with protons and neutrons heir respective quietly up up down and down down up quark compositions, and of course spacetime itself being bent by mass creating what we like call gravity, I think I just summarized modern physics).
@@JmO-ee1bi Photons do not exist, electromagnetic radiations is waves at all frequencies.
"super cool physics wardrobe" Bwahahahahahah so true!
He doesn’t know anything he’s just standing there reading
@@rogerc23 His tribe are constantly inventing ways to pretend they are more clever than everyone else, they even invent whole subjects as a pedestal to pose upon. Relativity is a hoax and quantum mechanics is a fiction created by treating waves as if they were particles and then claiming the particles really exist. Their vanity has reduced science to a belief system, like religion.
Great video. Feynman's explanation using the path integral in his little book QED is also fascinating and maybe more fundamental.
I have had the wrong explanation since 1971-when I studied physics and we were told the guff about the density of the glass being the reason -what a shame teachers then didn't have to have majored in their subject - thank you for putting me right. I'm mid 60's and still learning basics thanks to people like yourself !
They were right and this guy is just an idiot clone mouthing 'Physics Doctrine
Think why and how light is bent by a star's 'gravity'
And light is neither a particle nor wave and that's why we can't have anything nice
@RogerWilco99 metals have free moving electrons that aren't attached to any particular atom. That's why they're conductive. But electrons are also what absorb light. So by having tons of free roaming electrons, it creates an impenetrable wall.
Though it's worth noting that a thin enough sheet of metal actually will be transparent.
The density of glass is the reason that refractive indexes are different, as more dense materials have more atoms which also have electrons creating a new EM wave, so the net effect is a slower light beam, with the speed in the material being proportional to density
it's a well-known fact I learned in Ireland ... light slows down in water because it's thirsty.
And slower still in Guinness!
Dave Dewhurst it never reappear after a Guinness
@@JohnnyMotel99 Aye, and this is where we get the phrase "blind drunk"...because all the light we need to see by is absorbed by the booze. See how everything makes sense on the internet?
I'd figure in Ireland it was because the light was depressed it wasn't passing through whiskey.
But in Ireland, thirst is not quenched by water....
But why does the wave generated by the electrons move at a different speed?
And why does the new, slower wave does not generate a new wave, slowing down the light even more?
My guess. Because the speed of the electrons and the resultant wave is dependant upon the medium that they are part of. They are simply being agitated by the light.
i think because, unlike photons, electrons have mass.
My guess is that the wave generated is in opposition to the incident wave (moving in the other direction or even at other various angles) and the overall combination is impeding the initial wave.
@@pahom2 Thinking about it as 2 separate waves that you add together, is a human idea. What actually goes on is a complicated ripple in a pool called the electric field.
This is a completely new thing that I've learned today.. thank you❤
I think this is a much better hypothesis than the other two yet, with it, comes so many other new questions (as usually happens in science!!).
Thank you! I always find those explanation on the internet unsatisfying, but yours truly makes sense.
I feel so happy that I started to watch more and more of your videos. They are truly compact and spot-on, to me.
Thank you for putting this material on the Internet for free.
Congratulations on the muons experiment results.
I will spread the words that you spread :)
hahaha tell me how they measure the lifetime of a muon at rest? If they don't know the lifetime of a muon at rest, how do they know the life of the muon is extended by moving at velocity?
Please can you explain to me why the wave created by the electrons moves at a different speed 9:20 and also why the combined new way moves slower than the speed of light 9:43 ? Please I didn't find why...
What a concise and succint "get to the point" explanation! Thank you!
We've learned about this in optoelectronics class and had to derive it. I was very happy about learning about it. Although I forgot what is the exact reason for the light bending, as opposed to just "illusionary" slowing down. Snell law is all I remember :P
EDIT: Have watched your next video explaining the light bending. as expected, same phenomenon.
Please Bro can you explain to me why the wave created by the electrons moves at a different speed 9:20 and also why the combined new way moves slower than the speed of light 9:43 ? Please bro I didn't find why...
@@Khalid-Ibn-Al-Walid it does not. The reason is in innertia due to mass of electrons which means it takes time to accelerate them and waveform to "grow". So it lags behind the original wave.
Fermilab is wrong, it doesn't work like that, the speed is the same, it is the PHASE that goes back slightly, really the speed of light is the same, it is the appearance that the phase gives that makes us believe that it is going slower@@Khalid-Ibn-Al-Walid
9:05 ✌️ watch from here if u r in hurry , the actual reason I clicked on this video
Thank u sir ji ,mujhe smj aa gya!!
Thank you
If you don't have time for a 10 minute video you have issues
@@disgruntledwookie369 if you needed to watch this video to understand why light doesn't pass through other materials like it does through air, you also have issues..
Watching your demonstration of photons colliding with water molecules reminded me of another related topic, which is the time it takes a photon to travel from the core of the sun, to the surface and out into the universe.
Great video, thanks. One aspect you didn't explain(and I know it wasn't in the title) is why the light bends and then returns to the exact same direction. I'd imagine it is related to the way a water wave diffracts upon entering shallower water, but not sure how it would be explained in terms of your superposition of the two waves. Anyone?
i think this has to do with the frame of reference as light must always travel at a constant speed therefore it takes a longer path inside the material with higher refraction index and shorter path in air so it must change direction
@@Nightspyz1 The path inside water (or glass or whatever) doesn't matter. Light bends at the transition between different medium and only returns to the same direction if the surface of both transitions (air->glass, glass-air) are parallel. And Yes, that's not explained in the video even if both effects are connected.
@Cornelius Corcoran Perhaps a way to think of it is that as the light is coming in at an angle, the "inside" (left side of the red line in the video) of the light ray slows down before the "outside" as it enters the glass first. As the light maintains it's own speed and the slow down is just due to wave addition, on the way out of the glass where the light wave is no longer interacting with the electrons in the glass, the reverse is true with the left side of the light moving faster compared to the right, bending it back. Therefore, the light has the same velocity (speed and direction) when in the vacuum again (it helps having parallel sides on either side of the glass in the example to make that true). Refractive index and Snell's Law is key to all of this regarding determination of angles. I hope that's right / helpful as it's just my personal understanding!
@@a.k.1902 I think thats a common misconception. What about single photons? Do they have something like "inside" and "outside"?
What about light traveling through caesium gas with refraction index < 1and phase velocity > c?
PS: Feynman's path integrals may give an consistent answer to light bending.
Feynman says a singe photon takes each possible path (it may even fly around the moon to reach a screen 2m in front of the light source ;). But... almost all possible paths interfere with each other in a destructive way so the resultant probability along a straight line usually becomes almost 100%. However, because of different phase velocity inside glass (and other materials) the interference and thus the probability distribution also changes according the incident angle -- the phase shift on the inner side happens earlier than on the outer side.
The exact same principle is used by holograms on bank bills or many other today's uses. Holograms are partially covered mirrors which changes the probability distribution of reflected light to form a 3D effect.
Light approaching at an angle has a kind of angular velocity relative the surface. As it slows its angular velocity is kept but not its forward velocity. Why that happens is even more complex.
Outstanding explanation and another superb video Don, many thanks
Professor Fermilab --- I love you ( and I am 56 years old)! Please continue!
His name is Don Lincoln, Fermilab is the name of the lab he works in.
@@Vagabond-Cosmique yeshh
Thanks, Dr. Lincoln! Just curious, how does this explain total internal reflection? Why would a certain incidence angle be unable to cause the secondary electric fields? Will be glad to hear from you on this!
It not that “a certain incidence angle is unable to cause the secondary electric fields”, it is quite the opposite. The secondary electric field is still there in both cases it just might combine to give a different transmitted forward velocity or combine and give a reflected reverse velocity at the interface depending on the different medium/vacuum and angle. So, the answer is same idea from wave theory introduced in this video of wave superposition, but not only can they combine to have different speeds, but the combination can also be in the reverse direction to the primary wave depending on conditions. BTW great video and great question :)
@@aaronhoney2217 Any correlation to Lenz's Law?
@@dizzydinonysius This connects to Lenz’s law which relates to Faraday's law of induction by giving the direction of the induced electromotive force . Maxwell's classical electrodynamics is the theory for classical electromagnetic wave most commonly used in Heaviside’s form of vector calculus containing four equations. (Which most people learn. Do a google image search of to see Maxwell's equations).
Two of the four equations are for electrostatics (1. Gauss' equation) and magnetostatics (2. Gauss' equation for magnetism) the other two are relate to electromagnetic dynamics , 3. Maxwell-Faraday equation (contain Lenz Law) and 4. Maxell-Ampere equation. The last two form a coupled equation to give the wave equation and contains the speed of light. Applying all equations to reflection and refraction problems give the wave theory explained in this video.
@@aaronhoney2217 Thanks
@@aaronhoney2217 But who uses 'wave theory' to explain the propagation of EM energy these days? Do they not teach quantum physics any more these days?
PS For a modern approach read the well known Richard Feynman. Yes, I know he is dead and he got some things wrong but he is much, much better than this dinosaur.
I'm so thankful for that video! It's priceless that you not only explain correct answer but start with debunking of the most Internet-popular explanations indicated theirs weak points. Thank you :)
He has a great ability to teach clearly.
One petty quibble is that there is not just one way to describe how physical phenomena occur. There not any explanation of anything, just different ways to describe it.
I wish I could schedule this to watch once every 6 months. Thanks for making it.
This is best explanation I've seen on this. The interaction with electrons that then create the own electric field that then interacts with the light waves electric field has that sense of model predicting reality well. -- Is there further reading I can follow up on?
I was taught about fifty years ago in physics that the atoms absorbed and emitted the light in same direction (the second wrong belief). For various reasons (doesn't explain speed of light in different materials very well) it never felt right. When I saw the title to your video, decided to get a better understanding of the process. Surprise ... I was taught wrong. This explanation makes much better sense.
Please can you explain to me why the wave created by the electrons moves at a different speed 9:20 and also why the combined new way moves slower than the speed of light 9:43 ? Please I didn't find why...
@@Khalid-Ibn-Al-Walid The wave created by the atoms isn't actually going anywhere. It is an electrical vibration. Light is also an electrical vibration. There is a diagram of the combined wave at 7:05. You can see that the top wave is actually considered to be stationary. Anyway, that is the best I can explain. I became an engineer because I knew I wasn't smart enough to be a scientist.
@@JaimeWarlock I saw the video many times again and focused on the 7:05 as you said and I think I finally got it ! Thank you very much bro !
( Btw me to I want to be engineer, in IT, I like physics but I find IT to be way more accessible and practical 👍)
Thanks a lot Fermilab! Can I clarify something with your explanation? Electron motion produces EM field oscillating at a particular set of frequencies. Yet, refractive index varies with wavelength smoothly (hence, rainbow is always a smooth picture). How does that fit?
May I put my two cents in as a holder of a BA in mathematics who has read a first-year college textbook in physics? First off, keep in mind that physicists are the only group of people in the world who don't know that the Big Bang theory is stupid. (Just kidding; they know it's stupid, but they need a placeholder for the expansion of Space.) Secondly, your question appears to relate to quantum theory as contrasted with classical theory. As Dr. Lincoln said, physicists believe that Light sometimes acts as a wave and sometimes as a particle. That's my input, which might not even be worth two cents, but read the third paragraph of the Wikipedia article "Electromagnetic field". Have a glass of water and two aspirins nearby.
Electrons motion can have different frequencies depending on the electric potential binding them. When bound to a single nucleus (a Hydrogen atom), they produce a certain set of frequencies, probably what you were thinking of. However, when bound to a crystal lattice, such as inside a piece of glass, the potential is more complex, and allows the electrons to take on a continuous range of frequencies. It's a rough explanation but I hope it helps
I am far from satisfied as he was again telling a half story and trying to evade the real problem. People tend to play the wave-particle duality whenever it's convenient to them, without giving any accurate (though there's an extremely rough one) criteria when to use which. If you want to explain everything with wave, just keep consistent by viewing every glass quark as a de Broglie wave... Or alternative let's put it this way: try explain what happens when a single photon EM wave shoots into the glass from vacuum.
@@avivschwarz8513 Media like water and glass are not crystal lattices, yet they show a rainbow pattern of refraction. And yes, in metal-like lattices there is "merging" of electron energy levels into bands. But that's not universal either.
@@albat6538 I'm curious; why do you think the rainbow is a continuous (smooth) spectrum of colors? Surely quantum theorists have shown that only certain EM frequencies have the requisite energy amounts to produce a photon (?) of light. Forgive me if I'm showing my ignorance.
Thank you! I graduated physics department, but for the first time I come to understand why light travels slowly in water. Thank you for your nice video!
The speed of light is not constant; bye, bye, Einstein.
@@melvynobrien6193 Einstein said the speed of light is constant in a vacuum, which it is, and is also reinforced by the explanation in this video.
Please can you explain to me why the wave created by the electrons moves at a different speed 9:20 and also why the combined new way moves slower than the speed of light 9:43 ? Please I didn't find why...
This guy is a positive influence on my quality of life.
Neatly explained!
Thanks professor!
I really enjoyed it. Indeed, your explanation encouraged me to pursue studying physics academically.
Feynman's lecture (Volume I, Chapter 31) is better.
Please can you explain to me why the wave created by the electrons moves at a different speed 9:20 and also why the combined new way moves slower than the speed of light 9:43 ? Please I didn't find why...
@@carmelo665 Please can you explain to me why the wave created by the electrons moves at a different speed 9:20 and also why the combined new way moves slower than the speed of light 9:43 ? Please I didn't find why...
@@Khalid-Ibn-Al-Walid In short, the interactions of light with atoms (absorptions and re-emissions of photons) cause a net phase shift and the apparent speed of light as c/n. The speed of light in the "vacuum" within the glass remains constant.
In Feynman's words, “It is approximately true that light or any electrical wave does appear to travel at the speed c/n through a material whose index of refraction is n, but the fields are still produced by the motions of all the charges - including the charges moving in the material - and with these basic contributions of the field traveling at the ultimate velocity c (Feynman et al., 1963, p. 31-1).
@@carmelo665 Thank you very much for this answer ! But Why does it cause a shift in the wave ? I agree that it does modify the wave, but why it does Reduce it's size ?
I am not awed.
Your explanation is elegant
and elegantly presented.
So, I am inspired.
The Floyd T-shirt is also a nice touch.
Great explanation, but it would help if you can show graphically how two waves of different speeds interfere or combine such that the resulting wave has a slower speed
You can observe a similar effect with sound waves. If you've ever heard someone tune a guitar by tuning an open string to the one next to it (by pressing it to a fret), you'll hear the difference in frequency between the two strings as a slow, undulating frequency, which corresponds to the difference in frequency between the two strings. The person tuning the guitar will be looking to reduce this effect to zero, which means the two strings are at the same frequency.
9:18 "but the waves from the electrons move at a different speed"
But why is that ?
Isn't the space between atoms just a vacuum ?
it's because electrons and photons have fundamentally different speeds. this is because the electrons interact with the highs field, which pervases all of space so the space between atoms still has the higgs field. photons don't interact with the higgs field and this is seen because electrons constantly and randomly change their spin direction (indicating an interaction with the higgs boson, the constituent of the higgs field) while photons have one consistent spin direction which means it is not interacting with the higgs field.
@@leemaina8170 Perhaps Higgs mechanism could be used to explain why electrons don’t vibrate at speed of light, but this has nothing to do with EM wave that their vibration produces, which always will propagate at the speed of light. Think of a simple radio antenna as an example. Thus, there should be no difference between speed of external light wave, and internal EM wave produced by particle vibration in the same medium.
Rather, I would point out two problems I have with that cool animation @7:10:
First, frequency of resultant wave in glass seems changed while wavelength is same as before, which is quite the opposite from every physics book (wavelength should decrease with speed).
Second, using Maxwell’s equations, Dr. Lincoln explains in “Why light bends in glass” that bend is caused by a fact that resultant electric field decreases in glass, while animation here shows that it oscillates and can actually increase at times.
I think that's a mistake, both EM waves move at c, but their superposition at < c
Electrons have mass, they can not move at speed of light. Because they move, they generate electrical field opposite.
I have the same question. If somebody could explain it, i would appreciate it very much.
Awesome channel! Great choice on the t-shirt for the topic!
Thank you for explaining this. All those years of physics classes didn't help me but this did!
Please can you explain to me why the wave created by the electrons moves at a different speed 9:20 and also why the combined new way moves slower than the speed of light 9:43 ? Please I didn't find why...
@@Khalid-Ibn-Al-Walid Good question, but I don't think they went into an extreme depth in this video. For your second question, it's kind of simple. The "oscillated" field created by the electron is slower, so that would slow down the original light temporarily, until it speeds up again after passing through the object.
@@thechannelofknowledge5145 Thank you very much for answering ! But please why The oscillated field created by the electron would slow down the original light ? I can understand that if the field created by the electron is slower then it's light will reach us slower (the light of the electron) and it can also modify the structure of the original wave, but why would it slow the original wave ?
@@Khalid-Ibn-Al-Walid Sorry if I do not give a good or correct explanation, but I think that the field created by the electron isn't exactly light. I believe they said it is an electric field. If it was an electromagnetic field instead, the original light wouldn't slow down, as photons don't interact with each other.
Anyways, the electric field would start slamming into the photons, causing them to slow down. Since photons are restless, they would speed up again after passing through the container.
Since I do not have a correct answer for this, it's more of what I think that why the light slows down.
Can you elaborate on when you say "Light still moves at the speed of light in a vacuum, but the combine wave moves slower"
This implies that there is still a wave of light moving at the speed of light thus exiting before the combined wave. If that were true then a "light speed wave" would be detected before the combined wave exited the material.
Oh yeah good point; I wonder if that happens
It's easier when they use the formulae - in speech & writing it's hard to distinguish "THE Speed of Light", ("C", the constant defined as the speed of light in a vacuum), and "the speed of light" - (the speed of that light right there going through that prism right now.)
"The speed of light, C, does not change, but the speed of the light in the prism moving at "C divided by the refraction index" is moving slower.
Note that the refracted path through the glass is shorter than the straight line of the original light direction. The travel time of the slower light through the shorter path in the glass is exactly the same as the time of travel that would be at the speed of light c through the straight path.
@@ozachar Don't the outer portions of a convex lens cause the light to bend toward the thicker center, taking a path that goes through more glass? (Not an optics guy, that's why it's a question, not a statement of fact)
@@muninrob I still don’t get why the « slowed » wave will change its direction by an angle depending on the material itself… what have I missed in this otherwise very clear explanation of the light wave deceleration ?
A slight irony that in a video explaining the real reason why light travels slower through a transparent reasons has little animations of atoms as mini solar systems with electrons orbiting the nucleus rather than inhabiting orbitals. I guess we are never going to get away from that imagery...
you wanna look at a huge orange blob that moves and/or a ball with blue dots in shells?
what do you mean inhabiting? as in going so fast that it looks like rings instead of balls ?
It would be difficult to draw an orbital shell that stretches across the universe. Let's just agree that all drawings are stylized.
It's ironic that you are using the word "orbitals" while trying to get away from the solar system model. (yes I'm aware that orbital is the technical term)
It isn't even like the Solar system - nuclei aren't even in the foci of electrons' orbits!
Someone on the internet said, this is a poor explanation for light slowing down in water. But, I found this to be a best explanation so far.
Dr. Lincoln is great and this video was, once again, fantastic. How does the light not lose energy when it causes the electrons to move? From where does the energy that is driving the electron oscillation originate? The light leaves at the same wavelength and therefore the same energy.
Consider what it means in this context for the light to lose energy. It means the wave behind the material has a lower amplitude than in front of it.
The wave behind the material is the sum of the original wave and the response wave. If these are in phase the result will have _higher_ amplitude. Which would mean we extracted energy from the material.
If these are in some way out of phase we can keep the overall amplitude and energy the same. And since the electrons in the material just move up and down, no net work has been done. The electron is accelerated by the E-field potential, gains energy, and falls back down, giving the energy back. (You can get into more detail because there is a static potential from the proton and a moving potential from the wave).
If they are out of phase such that the amplitude is _lower_ then we have deposited energy in the material. Light has been absorbed and the material heats up.
We call a material "transparent" when it does not (significantly) absorb the light we are working with.
0:18 That's one of the reasons why I'm subscribed to Fermilab.....
Five questions:
1. How does light change its direction?
2. If light slows down due to the cancellation of two waves does that mean light can speed up as well?
3. Does that mean if u change the placement of atoms the lights direction will change?
4. How does light get blocked by opaque object then?
5. If most of the atom is empty space, how is it so common that it hits an electron?
I'm no physicist, but here goes:
1. I think the question here is 'why does light refract through a medium?' (you should watch a video on this - very simple)
2. No. Firstly, I think you mean 'superimpose' rather than 'cancel' as for two waves to cancel it implies that the wave height has become zero. It was stated in the video that when two waves of different speeds superimpose, the resulting superimposed waves net velocity is 'slower', as for why this is exactly, I'm not sure, but it would imply that it does not ever become 'faster'. The speed of light is calculated by [wavelength * frequency], take it from there... maybe...
3. The light will either interact with the electrons of the atoms or it won't. This interaction will only slow light, not change its direction (refer to 1. 'refraction').
4. It isn't blocked, but reflected, some light will get through. A solid material reflects the wavelengths of light that give it it's colour, e.g. a red brick absorbs all wavelengths of light, except for red wavelengths, which it reflects to your eyes.
5. Electrons act as particles, but they also act as waves (wave-particle duality). The light is interacting with the electrons electromagnetic field, which extends past what can be thought of as the physical electron itself.
Hope this at least helped you onto the right path towards getting fuller answers to your questions.
Never stop asking questions :)
2. czcams.com/video/uh-e4UFwzqE/video.html
@@fadair
1. No, I meant why does light change its direction according to his theory.
2. Okay
3. U gave the answer to this question in 4
4. Why is it reflected and why can't the light pass through the object if they are both made of atoms, e.g. if coal and diamond are made of the same element then why does light get reflected from coal and get refracted in a diamond block.
5. Okay
Another question popped up in my mind:
How does quantum particles form a colour?
@@samerator4233 Here's a little common sense intuition,
You can not see any object in the universe without light either reflecting off of it, or refracting through it.
And, you can not tell if light exists or not without the presence of objects, because in order to see light, you need it to hit objects, without which, you can't see light.
Sunny shah That definitely helped me answer any of my questions and I definitely did not know that -_-
It is good to understand that phenomenon at last. I remember the physics teacher talking about it 40 years ago but could not gave us a satisfactory explanation...Thank you..
Please can you explain to me why the wave created by the electrons moves at a different speed 9:20 and also why the combined new way moves slower than the speed of light 9:43 ? Please I didn't find why...
Hello, I have a question: is there any way how to seperate the two waves in the glass (I mean, the field of the incident beam and the secondary wave produced by electrons)? Can this combining of the primary and secondary wave in a material be deduced from Maxwell's equations? Thanks
Thank you sir, i was wrong thinking in one of those false explanation but now i know which is the correct and i know why
No u don't know why because he doesn't explain it
can you explain how light moves through a photography lens and how each specific type of coating that is used alters the light?
I've always wondered what exact chemicals they use for lens coatings and how that increases the total amount of light that that transmits through the glass.
it took me 18 years of school of distorting this concept, and 10 minutes of this guy fixing it all at once
Brilliant! I always wondered and even though I knew them to be wrong your first two erroneous examples was everything that I ever managed to suggest.
Please Bro can you explain to me why the wave created by the electrons moves at a different speed 9:20 and also why the combined new way moves slower than the speed of light 9:43 ? Please bro I didn't find why...
I just wanted to know this yesterday and found both wrong explainations... How can it be that you are uploaded the answers to the questions I google every time?! xD Thank you for the awesome video tho
His brain is entangled with yours
Spooky action at a distance. 😃
@@azelectrical9093 google more stuff, i want more episodes
illuminati confirmed
because google is recording your information so it gives you more "tailored" results whenever you search in a google sphere website or a website that bought your information from google.
Whenever I teach trig class, I always include a lesson on Snell's Law. Since Snell's Law is just sine functions and algebraic manipulation it is a great application of trig to include. But beyond that, everyone wonders why that stick in water looks like it is bent. Even most of the super math hating students seem to enjoy this topic.
This is why I hated math and loved physics.
Many teachers fail to include real world applications and just teach the calculations and formulae.
In my opinion it should be standard to at least name the top applications for any mathematical method you are about to teach to give students a chance to connect it with the real world.
@@boooster101 But then you have to consider which applications would be of interest to the student. This is why I think even algebra (let alone trig) shouldn't be as widely taught as it is. Sure, it has applications in life -- but to whom? And which? And that goes for lots of other school subjects that wind up a waste of time for most people.
Nice video. So, the generation of the electric field when light passes through a non-vacuum media will consume energy. How does this affect the light? Will it decrease in intensity? Is this energy somehow "recouped" when the light exits the medium and enters vacuum again?
Yeah, the induced electric field inside the medium causes some energy loss. This is transformed into heat, which is simply the jiggling of atoms (the same jiggling that generates the second electric wave to begin with). The exit wave will have the same frequency, but a slighty smaller amplitude, thus a drop in intensity.
@@KingIsulgard that’s not how energy in photons works. Higher energy means higher frequency / shorter wavelengths. You can’t apply the concept of “amplitude” to light waves. (Don’t take my word for this, look it up.)
So this also has me scratching my head: how can light lose energy but still maintain the same frequency? And why would the frequency influence the speed of light?
If that were true then light would travel at different speed according to its frequency. But it doesn’t, C is constant for all frequencies.
Light is massless so if it’s not losing energy through it’s frequency then it’s not losing any energy at all.
I’ve just started investigating this specific topic but my theory so far is that the interactions of the electromagnetic waves in light and matter causes some warping in space-time similar to gravity in general relativity, which causes time to dilate.
@@Yogarine Light speed is the same for all frequencies in vacuum. Matter slows down light of different colors in different amounts, hence rainbows (and dismay among optical fiber engineers).
The light doesn't lose energy due to the slowness (and the description in the video is a bit superficial, not even talking about quantum mechanics). Absorption and hence energy transfer to the medium happens by losing photon, not by changing their frequency.
@@Yogarine you can't use "amplitude" for _photons_, NOT light _waves_. The amplitude is an indispensable part of describing a wave. The energy delivered by the wave per second will be proportional to its frequency and the square of its amplitude. In case of a light wave, this amplitude is simply the "size" of oscillations in the electromagnetic field - the larger the wave, and the more it wiggles, the more the energy carried by it. More mathematically, an EM wave can be written as E = E0 • sin(ωt). The amplitude of this EM wave is E0 - the maximum height of the wave. Frequencies manifest themselves to our senses as colours, so if you change the energy of a light wave by altering the frequency, you'll see it changing colour. But you could also change the amount of energy delivered by increasing or decreasing the size of the oscillations, while keeping the frequency constant, in the electromagnetic field. That simply amounts to seeing a brighter or dimmer light of the same colour.
Here's a nice explanation from a freshman physics textbook to help you out. Follow through the maths to completely understand what's going on here. (In case you feel tempted to "debunk" me before going through this, here's the first line from the learning objectives of the chapter: "Express the time-averaged energy density of electromagnetic waves in terms of their electric and magnetic field *amplitudes*")
openstax.org/books/university-physics-volume-2/pages/16-3-energy-carried-by-electromagnetic-waves
Light itself doesn't change. Its wave superimposes upon the generated electric field of the electron. The energy of the superimposition is the energy of the light minus the energy of the generation of the electric field (the work required to jiggle the electron).
Thus when the light exits the medium the superimposition ceases and light is free to be the Universe's speed limiter once again.
Physics is essentially the study of how light interacts with matter and how matter interacts with itself. Light and matter are fundamentally separate entities so when we speak of light "changing" within a medium, we are really describing *the interaction* between light and matter. Once you factor in gravity fields generated in spacetime, the interactions become more than a little wonky...
I really had a fun time watching this video! It gave me a true explanation to as why light slows down in materials. I had a slightly hard time understanding this (since I'm 14), but just with some focus, I got to grasp on it!
Also regarding photon absortion and emission by atoms, they don't occur at specific time, but much more random, so if photons were absorbed by atoms, you could get an material that would glow after you switched the light source off, as photons would still randomly be emissioned by atoms.
Alen Gabric yes and the atoms would only emit at frequencies allowed by their electron orbitals... and btw what you are describing does exist, it’s called fluorescence :)
I was taught the molecular absorption delay nonsense at uni. Good to hear the propper reason now. Could you combine a photon with a sufficiently large electric field to slow it down to very slow speeds or even to rest?
There are crystals that can slow down light enough to trap it
Thank you. I pretty much understood that explanation, which is a compliment, because I REALLY struggled with Physics in college.
What are the properties that allow some solid materials to be transparent or translucent as opposed to blocking light as most solid materials do?
Missing from the explanation:
1. The speed of light through the medium varies continuously with the frequency of light somewhat defying quantum effects. Nice to use parallel sheet of glass but next time try a prism.
2. What is the atomic interaction with the molecular structure of the medium that makes it transparent? Obviously that structure has a direct effect on the speed of light in the material and its relationship to the frequency is also a mystery. You left out the part when showing index of refraction in those tables that it is measured with respect to a specific frequency of light I believe in the yellow range.
3. And since the electromagnetic frequency ranges quite a bit, stretch out that spectrum a bit to see what happens. I'd like to see photons explained in the radio frequency range.
Write a paper on it.
To answer your first question, it is because different frequencies of light have waves that have...well... different frequencies. since the velocities of the wave from light and the electric field are "added" together, different frequencies of light would yield different final speeds of the wave.
@@leemaina8170 This is the same preposterous claim made by fermilab. There is not only no math to support this, but the Fourier transform is the best piece of math to refute it. It is an illusion to be caught up in an apparent identity that sin (a+b) is the same as sin (a) + sin (b). The first form is definitely a sine wave but the second form is what you actually get when you add two sine waves of frequencies/wavelengths a & b together and that is not a simple sine wave with a single frequency. The deception is an attempt to equate the two equations. Not only does the math refute this but a glass prism or diffraction grating will also split apart the frequencies which were never combined in the first place. Interaction with electrons at the atomic level can absorb some of the energy and release it as light at a lower energy level/frequency. My assertion is that the speed of light through matter is a function of both the energy level of the light and the mass of the material it is traveling through. Light travels slower through higher density glass and the speed difference is exaggerated based on the energy level (frequency) of the light waves. Lens makers use this trick with compound lenses to reduce or eliminate chromatic aberration. This kind of infers Einstein's curvature or warping of space at the atomic level. But I left physics a long time ago and those equations are not on the tip of my tongue. Can somebody else write that paper?
If he did all that the video would be at least an hour long.
Great explanation! Would this then mean that the slowness is directly correlated to the number of electrons in the atom? Does the atom itself play a part? How far away from the atom must the light be to not be affected by the effects of the resistance from an atom? Could this become the definition of things ‘touching’ on the quantum scale?
I can tell you this much: the electrons are part of the atom and the chemical state of the atom as well as the number of electrons determine the degree of interaction. The spatial range is definitely on the scale of 'touching'. Touching in any case is electrostatic interaction.
If you think about a wave passing one atom... do you think it is possible to measure a change in the time tavelled for a photon for a single such event? Don's explanation (with reference to refractive indeces) is clearly meant to describe light passing through continuous matter.
Think about how the coupling of coils in a transformer varies with the core material. Now consider that at wavelengths like those of light, it wouldn't particularly be electric conduction electrons that would provide the coupling (as in the transformer), but electrons held in various degrees of tightness according to their energy state.
Not many ppl would watch this video and they would be under the assumption that what they were taught at school was right. I hope many educators take a note of this video. Thank you from an educator.
Thank you for the simple and wonderful explanation!
It was mentioned that when two waves of the same speed superpose, the speed of the resultant wave remains the same, only the amplitude is altered. It is only when the speed of the two superposing waves are different does the resultant wave have a speed different (less than) from either. When exposed to an electric field oscillating at a particular frequency, the electron feels a force due to this field and oscillated along with it. If so, both these oscillations have the same frequency and hence the external field and the field originating due to the election oscillation have the same speed. Then, wouldn't the resultant field in the medium have the same speed as the original one?
But frequency is not speed, so the electrons in the glass vibrate at the same f but do not propagate with the dange v
Exactly my thought Rohan!!! I don’t agree with the explanation in this video (just yet)!
Thats the video wrong axioma, I think. Interference dont change the speed of the beam.
Was that an explanation? - I'm not sure
it may have something to do with electrons having some form of natural frequency in the way they can be moved by the electric wave of the passing light... similar to pendulum or spring natural frequency ....
I think that can also be explained by Maxwell theory. Coz the velocity of light depends upon the inverse square root of permittivity and permeability of the medium.For different media they have different permittivity and permeability.
That still is complicated terminology. I’d say the best explanation is mass. Electrons have mass and so its electron field go slower than the speed of light in comparison to a photon.
The photon’s speed averages out with the speed of the electron, making a slower overall speed than the one the photon began with and once it is out of range of the electron field by the matter, the speed it will return to.
It is.
Great video!!!
If you had two synchronized clocks, one in a medium and one out of the medium, but in proximity, would time dilation occur, and at what scale?
The speed of light is different for different wavelengths of light. Nutrinos pass through without slowing down.
@@MrElvis1971 Just to be clear, on your first sentence, that only applies in a medium. All wavelengths of light travel at the same speed, of C, in a vacuum.
@Hagakure42 yes, in a vacuum all EM radiation travels at the speed limit C. Not sure if a particular wavelength of light experiences time in a medium as C is still unchanged, it's just light isn't travelling at C.
7:04 "that's just how it works." I'm good with that!
glossed over constructive interference with electrons :p still can't work out the speed limit. Thank you for the video!
isnt it destructive interference ?? im confused
At 4:43, this is the picture of a prism like on your shirt, so can you do another short video on how the prism works?
If there were a material with a sufficient index of refraction could one experience relativistic effects moving through it, or would you be unable to at enough speed and what is that relation like
@Fermilab I have a few follow-up questions. What I got from your explanation was that waves traveling with different speeds can combine to give a wave that travels at a third speed. And in this case, the two waves being combined are light traveling at a speed c and an oscillating electric field generated by the interaction of the light's oscillating electric field with the matter's electrons. Did I get that right?
My questions are these: 1/What is the speed of the generated oscillating electric field? And why it is not "c"? 2/How do you calculate that speed from theory? And how do you measure it in an experiment? 3/How can two oscillating electric fields of a different nature add up? And what is the nature of the combination? Is it also light?
They are both electromagnetic waves so it doesn't matter that they are "of a different nature." They add up like any other waves would add. The combination is light in this case.
@@kenbob1071 Hi. I had followed up to your comment three weeks ago, and now circling back I find your reply has been highlighted and my follow up is gone! Do you know how those two things happened?
Did not explain the change in direction.
But very well presented. Thanks!
So many videos on that already. Search for refraction Huygens principle
This is because one side of light front which enters the "glass" first - gets already delayed, while the other side which is still in "air" - isn't delayed until it enters "glass" after a while, and this leads to light wave front being seemingly "turned" to the side where delay already happens.
This is similar to a car with brakes applied asymmetrically (left-right sides not equal) turning to the side where brakes are more effective.
@@rvrss7192 i still feel like it should have been addressed in this video since he mentioned it and used the the change in direction to prove the other two theories wrong?
@@derekwallace1955 Yes this video was more about disproving other non-consistent theories and how and why speed reduction happens, while also important direction change phenomena was generally omitted.
On the other hand, direction change aspect is less complex and it's explained in many sources on internet.
where to begin...
this video has taken many shortcuts in explaining the relations between the two systems and how it affects them.
to better understand this, we need to talk about d'alembert's equation and its dispersion formula and how it describes a wave.
so d'alembert's equation is used to link together the spatial and temporal variation of a wave.
and from this equation comes the dispersion relation that links together the frequency, wavelength and celerity of a wave.
to access to this equation, we use maxwell relations to describe this phenomenon. when we consider those relations in the void, we get the relation: c² = k²*w² where k is the angular frequency and w its wavenumber. (just google it, ain’t that hard to understand)
thus we can see here that the celerity is proportional to its angular frequency. however, as explained in the video, when light enter a medium, there are charges that can be influenced by the electric field, especially electrons that are 2000 times lighter than their counterparts protons.
thus, when light comes in this medium, we assume that only electrons are affected by the electric field.
we then apply newton's second law to those electrons to find a relation between an electron's speed and the electric current
(we also neglect the magnetic field of the light as the electron is relativistic thus the force from the electric field is much greater than the magnetic one see lorentz law to better understand this )
and then by generalizing this equation to all electrons in the medium, we can get an induced current from the electric field of the light.
( the current is j = n * v with n the volumic density of charges and v the celerity of electrons)
then when we put this back to maxwell ampere local equation, we introduce a new component: the induced current which thus gives us a new dispersion equation: c² = k² * (w² - wp²) with wp the plasma oscillation and thus we can see that from the value of wp, we'll get a different value of the celerity of the wave.
thus we can say without a doubt that they are wrong in their explanation as it is not the combination from the electric current of the electrons and the wave that makes it slower but the induced current inside the medium that modifies the dispersion relation thus modifying its speed.
or in simpler words, it's the movement of electrons that changes the frequency and celerity of the light, but not the combination of the two electric current. (also on a side note, trying to represent the electric current of a dipole using a sine wave.. funny thing you did there 😅)
and finally, as for the reason why the light has a different angle, well you can easily explain this using two waves parallel to each other crossing the medium in phase with each other. as they have the same wave plane, you can easily come to the relation between the refracted angle and the incident angle using some basic geometry. i recommend you to see the demonstration of descartes second law if you want to better understand what i'm talking about.
Gosh that was a lot more intricate than I imagined. Whoever figured this one out first?
He captures my whole attention span. What charisma, personality and articulation! In my opinion this particular speaker is a "physicist at 💙. He exudes the affection he feels towards such a venerable and indescribable science. Thank you. Ana Abreu.
He just described the Ewald-Oseen extinction theorem totally wrong.
en.m.wikipedia.org/wiki/Ewald%E2%80%93Oseen_extinction_theorem
You can find the falsification explicitely:
"...each of these waves travels at the speed of light in vacuum, not at the (slower) speed of light in glass."
The explanation of Don Lincoln is just plain wrong, both in physics and math. "recombination" of 2 waves travelling at different speeds is just a stupid, impossible idea. If the slower one is just not there yet (in a specific point where "the light" has already arrived), then how could it be added to the faster one?
@@PafiTheOne lmao, that guy has a PhD, there's a reason he works at Fermilab not you
@@rajdeepbhandari8969 It seems science is a religion for you. Do you undertand anything I wrote? If not, then please learn first!
Your claim is a fallacy:
en.m.wikipedia.org/wiki/Argument_from_authority
However your claim is not even an argument, only bullshit, because you also failed to state your relevant opinion.
en.m.wikipedia.org/wiki/File:Graham%27s_Hierarchy_of_Disagreement.svg
Your response is an ad hominem attack, while my comment was a falsification of a central statement of this video.
@@wolfie54321 That theorem is just an alternative mathematic description of wave propagation. It is compatible with Hygens principle, and that recognize particles, that's why it is preferred (sometimes) in describing wave propagation in substances. But applying Maxwell's equations with the measured (or calculated) average electrical properties of materials is simpler. Both are correct in normal case. But at very high freq neither of them are accurate (in materials).
Very nice explanation! In some sense, could we say that the matter acts as a wave impedance or resistance against light propagation?
This is probably the best explanation for the slowing down of light inside a medium. They should show this in every physics class!!! :)
Excellent explanation! Also, that t-shirt represents two of my favourite things: Physics and Pink Floyd.
Which in this case are both corresponding to the waves :D - Brilliant.
@@Kombivar, and both give you perspective on "Time". 😉
Thank you for the explanation. I have a question. Can we make the ame effect if we generate electric field that emulates these electrons fields?
yes, people are doing that with metamaterials! they can even make materials that "break the rules" about direction of the light bending
@@nmarbletoe8210 thanks for the explanation.
So we can view the relative speed of light, not by watching it move over the course of time but by viewing the reflection of the light in any given moment. That just blew my mind. I never thought of that. This seems to be a window into the hypothetical "external" perspective and the limitations of the internal one. Simply the way our language emerged and evolved reflect the internal perspective in such a way to actually make it difficult to avoid certain biases when trying to comprehend or understand the universe. We are forced to discuss these topics from a perspective that is purely utilitarian, using words to describe motion, action, etc. (including those words) that take a perspective (merely "deemed" optimal for survival under specific conditions through the emergent process of evolution) for granted as somehow objectively real. It's rare to be able to demonstrate such a thing so simply as putting a pencil in a glass of water. Amazing.
You made a grand assumption that "we can view the relative speed of light [...] by viewing the reflection of the light in any given moment." That is nonsense. Not only is it wrong, it seems your understanding of light is incorrect. We cannot "see" (using other photons) the "reflection of light". So based on your assumption, everything you said after that (i.e. your next assumptions about a "hypothetical 'external' perspective") is also incorrect. Reality & science are not things to play with, using words.
Great video. But sir, can you make a video also about the impossibility to measure the one way(incident) speed of light due to Einstein clocks synchronization problem.
So it's kinda like dropping a magnet down a pipe.
Or throwing a hotdog down a hallway.
Exactly the same, one's electric fields, one's magnetic
That's an amazing analogy. Kinda makes sense.?. Got me thinking 👍🏼
Yo analogy is preety much apt.
Pretty much, the inducing of a secondary wave is what makes it similar
Super easy explanation.
That is the last reason I would ever have assumed why light is slowing down. So crazy 🤪
I'm amazed by that
There is wrong in him ,all em wave travel in same speed ,when he saying about the electron hear it carefuly
@@umermukthar4959 If what you said was true, then it would be impossible to light to have an effective velocity in a material medium which was slower than light. Then, electromagnetic waves induced by electrons when they're moving translationally because of light's electromagnetic fields's influence in them can't have the same velocity as light ones. They need to be instead slower than light ones.
Makes perfect and intuitive sense. Thank you!
What I draw from all Dr Don’s videos is that a big bloke can also have a big brain, both literally and figuratively! 😊
Thanks for the great explanations Mr. Dancing Hands
this video has taken many shortcuts in explaining the relations between the two systems and how it affects them.
to better understand this, we need to talk about d'alembert's equation and its dispersion formula and how it describes a wave.
so d'alembert's equation is used to link together the spatial and temporal variation of a wave.
and from this equation comes the dispersion relation that links together the frequency, wavelength and celerity of a wave.
to access to this equation, we use maxwell relations to describe this phenomenon. when we consider those relations in the void, we get the relation: c² = w²/k² where k is the angular wavenumber and w its angular frequency. (just google it, ain’t that hard to understand)
thus we can see here that the celerity is proportional to its angular frequency. however, as explained in the video, when light enter a medium, there are charges that can be influenced by the electric field, especially electrons that are 2000 times lighter than their counterparts protons.
thus, when light comes in this medium, we assume that only electrons are affected by the electric field.
we then apply newton's second law to those electrons to find a relation between an electron's speed and the electric current
(we also neglect the magnetic field of the light as the electron is relativistic thus the force from the electric field is much greater than the magnetic one see lorentz law to better understand this )
and then by generalizing this equation to all electrons in the medium, we can get an induced current from the electric field of the light.
( the current is j = n * v with n the volumic density of charges and v the celerity of electrons)
then when we put this back to maxwell ampere local equation, we introduce a new component: the induced current which thus gives us a new dispersion equation: c² = (w² - wp²) / k² with wp the plasma oscillation and thus we can see that from the value of wp, we'll get a different value of the celerity of the wave.
thus we can say without a doubt that they are wrong in their explanation as it is not the combination from the electric current of the electrons and the wave that makes it slower but the induced current inside the medium that modifies the dispersion relation thus modifying its speed.
or in simpler words, it's the movement of electrons that changes the frequency and celerity of the light, but not the combination of the two electric current. (also on a side note, trying to represent the electric current of a dipole using a sine wave.. funny thing you did there 😅)
and finally, as for the reason why the light has a different angle, well you can easily explain this using two waves parallel to each other crossing the medium in phase with each other. as they have the same wave plane, you can easily come to the relation between the refracted angle and the incident angle using some basic geometry. i recommend you to see the demonstration of descartes second law if you want to better understand what i'm talking about.
Much fun and i love the straightforwardness of the reasoning and presentation, but there is something i am missing. If the interaction of the light wave with the electrons is producing a second oscillating electric field that will be summed with the original light wave, wouldn't the second field have a constant phase delay relative to the light wave? That wouldn't produce a slower wave - it seems like the summed wave would be an attenuated (by the second field) version of the original light wave traveling at the same speed. I don't understand where the slowing comes in.
The phase velocity of the second wave is much smaller. That drags the phase velocity of the summed wave down.
I agree, I am missing something too. The only way that makes sense to me is if the induced electric field travels slower than the speed of the inducing field - which doesn't sit well with my understanding of physics. Dr. Lincoln said that (paraphrasing) "taking longer to traverse the medium is the same as going slower", why would the interaction with the induced electric field make the resulting superposed waveform take longer to traverse the medium?
In the link below there is a really good explaination with quantum field dynamics. It is less hard than it sounds. It comes down to this: When a photon enters the medium, the EM-wave interacts with charged particles in the medium. It shakes them around, and drags them with it. To do this, the photon has to give some momentum to the material. The cool thing is, that while doing this, the energy of the total wave stays the same, but the momentum goes up because the mass that is part of the wave goes up. But if the momentum and the mass of the wave goes up while the energy in the wave stays the same, the propagation speed must drop. At the end of the glass, (almost) all of the wave energy goes back into the massless photon, and it shoots off like nothing happened. Just learned this, and I think this explaination is very satisfying. journals.aps.org/pra/abstract/10.1103/PhysRevA.95.063850
I think it's because the wave isn't really a wave travelling though space but is a harmonic lateral stretching and compressing of space (expanding Universe on a sub sub sub sub sub sub sub sub sub minuscule scale). Somehow that =propagates= some minuscule distance quantum in the direction that it not lateral, and this is how it advances. I think if you do that for 2 of these wave interacting then that propagation rate will be reduced because one is stretching space but the other is simultaneously compressing space so the net is a smaller stretching (assuming stretching effort exceeds the compressing effort) therefore the effect reaches its peak later. I'm just thinking this as I type and it requires the work of drawing the 2 things opposing each other to determine whether the net results in slower forward movement but I'm thinking that it wouId. If it interests, somebody do that and confirm or refute the concept. I'm tired out and it's time for a mystery movie short now before sleep.
Light is slowing because the Speed of Light is NOT A CONSTANT. Period. All other answers are mathematical tricks.
Thank you. As an ophthalmology teacher, refraction is a deep part of our daily job. And this explanation is outstanding!
It stands out of the reality.
Don tried to describe the Ewald-Oseen extinction theorem, but in a totally wrong way.
en.m.wikipedia.org/wiki/Ewald%E2%80%93Oseen_extinction_theorem
You can find the falsification explicitely:
"...each of these waves travels at the speed of light in vacuum, not at the (slower) speed of light in glass."
2 particle based wrong explanation has been replaced here by a wave based wrong explanation. A fast and a slow wavefront can not be combined, since they can not stay together. This is not a physics model, just a wrong play with words.
The way of thinking of Don is fundamentally wrong, because propagation of light can not be modelled with infinite sine waves, that has no wavefront at all.
Most of the animations are also completely wrong, for example at 7:11, because it shows different frequencies, while the excited wave can not have different freq. from the source wave (as can be seen at 8:11). Not wavelength, but frequency determines the color!
Almost nothing is right here, except for the falsifications of the 2 particle based explanations.
Great explainer! I had that question for 40 years.
When electrons emmit the wave, what kind of wave is that? Is it a light wave, or does it have mass?
Moving electron generate EM (light) waves. Al moving charges emit EM waves.
@@wayneyadams if so, the EM waves from electron should have the same speed as light, so why the combined wave is slower? Also, why the wave from the electron cannot leave the material? So confusing.
people93 I don't know the effect distance of an electron but the reason for slowing when interacting with the electron's magnetic field is clearly shown (the sum of 2 sine waves out of phase). The TER will speed up between electron effects but then be affected by another electron which slows it again so the photon wave is not travelling at a consistent slower speed but is accelerating & decelerating as it approaches electrons and distances itself from them. The reduced speed is the =average= through the material. This is why it slows more i a liquid and solid (molecules packed tight) than in a gas (e.g. air) with molecules and their electrons much sparser and even faster in a vacuum (minimal molecules and their electrons). I deduced all this from the speed variation and interacting magnetic sine wave explanation, never read it anywhere so you best search & check that.
@@grindupBaker the sum of sine waves in the same speed, even if out of phase, should not slow down the resulting wave, only change other parameters as wavelength or amplitude
Finally, an explanation that made sense. Thanks a lot.
It still doesn't make sense to me
@@MrSpock-sm3dd There are 2 crucial bits:
1. 8:03 - 8:21 - the oscilating photons induce the oscilation of electrons. The oscilation naturally have the same frequency, but different phase and the amplitude.
2. A sum of sinusoids at the same frequency is another sinusoid at that frequency - always true. The proofs of that are a bit complicated, so You can just remember that's how it is. What changes is phase and amplitude. An animated illustration of that is 7:06 - 7:30. The original wave (2) moves at the speed of c, while the induced wave (1) is almost stationary (at least here, I don't know how is it in phisical body e.g. glass). The resulting wave (3) is the weighted sum of (1) or (2), weights being amplitudes. The larger the amplitude of (1) the more (3) resembles it, so also the slower it moves.
That's how I understand it on a basic level, although I wouldn't be surprised if there are some holes. If anyone would like to correct me or add something, please do.
@@MCMaterac Where I get lost is the assertion that the amplitude being changed somehow affects the speed. I don't think that's a property we observe in other waves, to include light. And there's no explanation in the video as to how we arrive at that conclusion.
@@That_Montage_Nerd You're right, that's a wrong assumption. I've just made a javascript simulation. If the 2 amplitudes are equal, it looks just like in this video and in relation to wave1 the resulting wave3 moves half as fast as wave2. However at other amplitude ratios the resulting wave moves on average at the same speed as the input wave that has higher amplitude, with additional left-right oscilations repeating each time wave2 has amplitude higher than wave1, the wave3 propagates on average as fast as wave2, accellerating and decellerating each period on repeat.
So yeah... the video (and my answer) doesn't seem to explain the matter well.
@@MCMaterac Bro are you Indian?
Excellent! It's always great to meander through one's day and end up a tiny tad smarter than what one was before coffee. With my new-found tad of smarts, I am now inspired to ask whether or not it is possible to measure the electric field of the excited electrons in the glass, when the glass is struck by light?
Amazing explanation! Loved the video.
Follow-up question: What affects optical density? Physically density would make sense considering there's more atoms per unit volume, and hence, more electrons per unit distance in the path of the light to cause the additive wave which is slower than light-speed. But turns out, physical and optical densities aren't exactly related. So what determines the optical density of a medium?
Sir could you explain why frequency remain constant in refraction
The frequency can remain the same but if the wavelength changes the equation will change for wave velocity, V=wavelengthxfrequency
Sorry but for me this video raised more questions than it answered.
First of all, why doesn't the "original" wave come out of the glass weaker? With no other information, I would expect all light entering glass to exit at two different times: the first, travelling at exactly the speed of light in a vacuum, made up of respectively the original light emitted by the laser and the independent light emitted by the photons (dimmer because it lost some of its energy to the electrons). And the second, the sum of both waves, arriving later because it was travelling slower.
Also, why does the light change path as a result of it slowing down? Going with your explanation, shouldn't the sum-wave move in the same direction, if slower?
I knew I didn't have a grip on it before, now I feel like I'm gripping it by the blade...
For the light changing paths question they made recently made a video on it,
You should read Feynman's excellent explanation in his lectures (warning, it uses some math, but you should read the text anyway):
www.feynmanlectures.caltech.edu/I_31.html
@open up eyes The video uses a moving graphic to show how this works. It shows the combined wave at the bottom. Try watching it again.
@open up eyes I would say yes, although at *trillions* of times per second. Doubt you can see that.
czcams.com/video/dd6IsYLCOZc/video.html
Thank you so much Dr. Lincon. You said the light wave causes the electron to move, which electrons and by how much? Does it shift every electron probobility density in the universe by a small amount, or only a small number which happen to be exacly along the light's path?
Please can you explain to me why the wave created by the electrons moves at a different speed 9:20 and also why the combined new way moves slower than the speed of light 9:43 ? Please I didn't find why...
Excellent! The one point glossed over is: Why is the wave slower just because the wave is superposed with another wave due to the motion of electrons? I would say that this motion of electrons represents polarization of the glass,, which is a self electric field that opposes the original electric field. This efectively weakens the electric field, and so weakens the restoring force for the wave equation