How Does Light Slow Down in a Medium, if Photons NEVER Do?

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REFERENCES
How Maxwell found speed of light: • Why is the speed of li...
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CHAPTERS
0:00 Einstein's Special Theory of Relativity
1:23 Skillshare: How to make animation like this
2:40 Why is the speed of light constant?
4:30 Speed of light changes but not photons
6:05 Why light seems to slow down
8:39 Quantum Mechanical Description
SUMMARY
Why does the speed of light change in different media? or Does it? How photons always travel at the same speed, but the speed of light changes.
In 1905 Einstein proposed the theory of special relativity. One of the postulates was that that speed of light is the same in a vacuum, the speed of light constant. But in other media, like water or glass, the speed of light changes. This is the reason we can see a bending effect when putting a straw in a glass of water, and the pattern of colors when light goes through a prism. But while the speed of light we perceive changes, the speed of the individual photons making up the light does NOT change.
Why is the speed of light constant. One way to explain this is to look at Maxwell’s equations. It states that the speed of light is due to the properties of space time. Since these properties do not change, the speed does not change. This is the logic Einstein used to formulate his theory of special relativity.
Any light traveling at an angle changes direction due to a change in its speed. So for example, when light travels from air into water, it slows down, causing it to continue to travel at a different angle, and this leads to refraction of the light which is the distortion that you see. But although the light that you perceive slows down, the photons making up the light are not slowing down at all.
Light is an electromagnetic wave. Electromagnetic waves are composed of changing electric and magnetic fields. Changing electric fields induce magnetic fields and changing magnetic fields induce electric fields, and together they make electromagnetic waves. One quanta of such a wave is a photon.
Unlike empty space, substances such as water and glass are made up of charged particles. Why charged? Well, they are made up of atoms, and atoms contain positively charged protons and negatively charged electrons.
Charged particles are affected by changing electromagnetic fields. We also know from Maxwell, that the movement of electric charges creates an electromagnetic field of its own. Specifically the movement of electrons, creates photons. Photons are light, the movement of the electrons emits a light of its own.
These are induced electromagnetic waves, and they are at various wavelengths. Most of these induced light waves cancel each other out, except the ones traveling in the same direction as the original light wave.
Each of these induced light waves, which again are streams of photons, travel at the same speed, c. This is called their phase velocity. However, what we actually see with our eyes is not these individual waves of light, but a mixture or summed up wave of light. In other words, the induced light waves interact with the original undisturbed light wave that entered the substrate. Since the light waves are at different wavelengths, there is constructive and destructive interference in parts of this summed up mixture of light. And this summed up light wave in glass and water, it so happens travels slower, that is, LESS than c, the maximum speed of light.
So the apparent speed of light, that is, what we see with our eyes, is the group velocity, or a summed up wave slower than c, even though all individual waves are traveling at the maximum speed. This is the reason that no acceleration or speeding up of individual photons occurs after the light leaves the substrate. In fact, the photons had been traveling at the same maximum speed the whole time.
Quantum mechanical picture: In a nutshell, the summed up group velocity of light, in quantum mechanical terms, is due to the superposition of all the various paths that the undisturbed light can take and its interactions with all the atoms of the medium.
#speedoflight
The photon takes every possible path and interacts with every atom, and all the induced photons created. And what we see as a result is the superposition of these paths and interactions. These photons by following every possible path ends up interfering with itself and other photons in such a way that it creates the net effect of a wave that travels slower than the speed of light. The result is identical to the classical picture I described earlier, but the details are different.

[@ArvinAsh]

Learn how to make animations like in this video by using Skillshare! The first 1,000 people to use the link will get a 1 month free trial of Skillshare skl.sh/arvinash07231

[@ogi22]

Very beautiful and succinct explanation how light speed is changing in a medium. And what i love the most about this one in particular, explanation about differences between classical and quantum mechanical approach. Thank you! 🥰 Terry Pratchett - 'Y'know,' he said, 'it's very hard to talk quantum using a language originally designed to tell other monkeys where the ripe fruit is.'

[@williamarnold9744]

Very nice explanations, both classical and quantum. Thank you.

[@3zdayz]

So I can make them wrong? Why would you keep showing a graph that has a maximum magnetic potential and maximum electric potential at the same time and zero at other times? You don't have both at the same time. You have electrostatic and electromagnetic. In a capacitor there's no magnetism in a inductor there's no capacitance. So the nature of the wave is never simultaneously magnetic and electric.

[@3zdayz]

I hit you inner too soon. I mean their legged by 90° one way or the other.

[@ogi22]

@@3zdayz I had the same thing too when i started to learn circuits😁 It's EM wave, not Volts-Amps graph in an oscillating circuit. Related, but two different things🙃

[@jelliebird37]

Arvin, I have had a lot of formal education in the sciences, and I have endlessly pursued understanding out of personal curiosity, and I have never seen anyone who can break things down and describe them in a way that makes them as meaningful as you do. In medicine, and similarly in other fields, the mantra of learning is “see one, do one, teach one”. When you can teach one, you actually understand. I want to thank you for being humble enough to be relatable. It’s easy to walk away from a genius, dazzled by the brilliance of things we can’t possibly understand. That’s what you get with ego. It’s *too* easy to walk away from someone actually understanding something they just taught us, and not appreciate their brilliance. You allow your viewers, your “students” to walk away with the ego you leave on the table, as if we’re the ones who just earned out newfound understanding. You are humbly, graciously, selflessly, *brilliant*. My goodness! Thank you!

[@ArvinAsh]

Thank you so very much! Glad you enjoy these video. Your poetic eloquence is rare.

[@algirdasltu1389]

also floatheadphysics is great too

[@aanchaallllllll]

0:12: 🔬 Einstein proposed the theory of relativity based on two postulates: the laws of physics are the same for every observer in a non-accelerating reference frame, and the speed of light in a vacuum is constant. 2:48: 🔦 The speed of light is constant and the same for every observer due to the properties of the vacuum. 5:33: 🌈 When light travels through a transparent medium like glass or water, the atoms in the medium are affected by the electromagnetic fields and start to move and vibrate, creating their own photons. These induced light waves interact with the original light wave, resulting in a slower apparent speed of light. 8:22: 🔬 In quantum mechanics, the slower group velocity of light in a medium is due to the superposition of all possible paths and interactions with atoms. Recap by Tammy AI

[@DanteGabriel-lx9bq]

This actually happens in biology too. You see chlorophyll, the organelles produced by photosyntheic plants, are quantum! That's because plants capture light to make photosynthesis happen. To make this; the anatomy of the plant needs to compensate for the multitude of roads light can take, and photosynthetic plants are extremely efficent at capturing light, because inside of them they have every road light can take, and thos they capture light at 11% efficiency, which can vary depending on wavelength, which is approximately around 400-700nm. Nature is always one step ahead.

[@4or871]

photosynthesis has an extreme high ratio between fotons which cause photosynthesis and which not. Seems to come through superposition of the photon, always a superposition possibility knows the right way. So 1 Qbit can determine all outcomes? not just for 0 and 1?

[@DanteGabriel-lx9bq]

​@4or871 I'm not really the expert in this, I study chemistry, so I don't really know.

[@lukebyer2592]

The following is a breakdown of the energetics of the photosynthesis process from Photosynthesis by Hall and Rao:[6] Starting with the solar spectrum falling on a leaf, 47% lost due to photons outside the 400-700 nm active range (chlorophyll uses photons between 400 and 700 nm, extracting the energy of one 700 nm photon from each one) 30% of the in-band photons are lost due to incomplete absorption or photons hitting components other than chloroplasts 24% of the absorbed photon energy is lost due to degrading short wavelength photons to the 700 nm energy level 68% of the used energy is lost in conversion into d-glucose 35-45% of the glucose is consumed by the leaf in the processes of dark and photo respiration

[@hmu958]

I've had it explained that it's entanglement of the metal ions, atoms, in the electron chain

[@erikwislinsky5961]

@@lukebyer2592that’s one hell of an in-depth comment. Props. Now I’m really curious about exactly how each wavelength makes it’s way.

[@FunkyDexter]

A few things: 1. The speed of light is not constant in general relativity, because gravitational fields are not inertial frames of reference. Locally observers still measure the same speed of light, but across larger distances they might see the speed change (it actually can assume ANY value depending on reference frame). GR supplanted the postulate "speed is constant" with "light travels on null geodesics". 2. Photons only exist as part of field-matter interaction. The EM field itself can take on any value of energy. Just think about the field generated by an accelerating charge, it forms a continuous spectrum depending on acceleration, which is not quantized. 3. There is a deep issue with your representation of the primary wave and the stimulated wave to show group and phase velocity (around the 8:00 minute mark): you're constantly talking about photons, but what you're showing here are two continuous waves interfering. Notice though how the resulting interference is comprised of wave packets (again, result of matter-field interaction). The description of phase velocity and group velocity makes sense if we're talking about continuous waves, but not if we're talking about quantized energy exchanges. I don't think you explain this clearly enough at the end of the video, you just talk about "quantum nature" without explaining how the difference arises. 3.5 Another important issue with the group velocity picture is that it only appears when the two interfering waves travel at different speeds. You kind of swept it under the rug first showing the two waves traveling at the same speed with a phase difference at 7:20, then you sneakily made them move at different speeds at 7:30. So even if the incident waves were to travel at c even inside the medium, the stimulated wave would not. So which is it? If you look up the definition of refractive index, you find that it is the ratio between c (speed of light) and the PHASE velocity (not group velocity). I.e., the speed of light is different. 4. If we want to use the photon description, we need to use another reasoning. The real reason photons are quantized entitities is not that they contain a minimum amount of energy (well, they do, but this is a result of another property): they have quantized angular momentum, hbar. This is a direct result of emission from matter: the difference between adjacent levels of the quantum number of angular momentum in atoms (ml) is exactly hbar. Since the "wavepacket" is restricted by moving at the speed of light in a vacuum and the angular momentum must always be hbar, the result is the famous relationship E=hf. Note that f can take on ANY value. 5. In a semi-classical way, we can assign an "effective radius" of the photon to account for the angular momentum (according to L=mvr, with m being the effective mass of the photon E/c^2). The angular momentum applies a torque on charged matter in a medium, which in turn acts to reduce the angular momentum by an amount hbar/n^2, where n is the refractive index. The effective radius though is the same as in free space: the only way for this to happen to conserve energy is for c to be smaller. On this point I reference the papers by Loudon, Padgett et al., and Mansuripur. 6. Related to the above, Maxwell equations are very clear: c changes because mu0 and e0 are not those of a vacuum anymore.

[@TimothyShevgun]

@FunkyDexter, I mostly agree with all your remarks, especially with the last one about Maxwell's equations. I am not sure whether it is reasonable to exploit the concept of the photons for the basic level explanation, or it would be well enough to rely on the wave nature of the light here. I suppose that some simplified explanation for wide audience can be made up via some simiriaty of permeability and permittivity to some viscousity. The larger the viscousity is, the more time it will require to make a 'wave splash' in it. Unfortunately, this video presents the wrong explanation for the original question. In pursuit of oversimplification, the authors have come to a complete mess. Basically, the concept of the difference between phase and group velocity is described inaccurately with rather an unclear example. What is more, this concept is not a proper explanation for the initial question. As a matter of fact, the PHASE velocity of the light in a medium is smaller than in vacuum. The quantity that remains the same in any medium and vacuum is the FREQUENCY while the speed and wavelength will change, the relation is as simple as c=f*λ.

[@davidrandell2224]

Light is a cluster of expanding electrons. “The Final Theory: Rethinking Our Scientific Legacy “, Mark McCutcheon for proper physics.

[@FunkyDexter]

@@davidrandell2224 Sorry, that book makes very little sense. It's unfortunately very lacking both in mathematics and basic physics.

[@davidrandell2224]

@@FunkyDexter A proton is a collection of 1836 expanding electrons and add a bouncing expanding electron makes a hydrogen atom. Electron mass (9.11)) multiplied by 1836 equals the proton mass (1.67). “G” calculated from first principles- the hydrogen atom- in 2002. All atoms and atomic objects are expanding at 1/770,000th their size per second per second constant acceleration: gravity; d=1/2at^2 major part of the gravity equation ( Atomic Expansion Equation, page 107). Gravity is simple Galilean relative motion with the chances that the earth is approaching the released object being 1 out of 2, I.e. 50%. Yet this simple fact- even as a “ thought experiment “- exceeds human brain power. SR wrong due to reference frame mixing and bad math. GR as well.

[@vaclavkrpec2879]

Thank you! This makes more sense indeed. Especially your 3rd point; the explanation by group velocity didn't make much sense to me---I asked "but in a medium, an impulse of light reaches distant detector _later_ than at the same distance in vacuum, right?" That difference doesn't really depend on wavelength in any way; so the explanation using wave interference doesn't seem to work... I also find your 5th point (explanation based on conservation of energy) a lot clearer than Arvin's explanation.

[@kjblalark]

I love your content as a 15 who is trying to study physics this channel helps me a lot

[@SailboatAqua]

@kjblalark awesome to hear! That's how I started! I'm 28 with a masters now. My advice for you is to take math classes seriously and make sure you are open to engineering. They make way more money!

[@SuperYtc1]

@@SailboatAquanot all about money thought. And engineers don’t make way more, maybe a tiny bit more after tax. But with a physics degree you can do many things, even specialisation into engineering. I say this as someone with a math degree.

[@hoseinayan942]

future scientist

[@kjblalark]

Thanks you guys for the support appreciate it

[@msislam6751]

This reminds me of me being 15 yr old 3 yrs ago and finding the channel

[@krwada]

This is perhaps the very best explanation of group and phase velocity from a conceptual view. I wish I had seen a lecture like this in my younger days at university. Maybe you can also explain, (graphically), what happens when a charged particle goes faster through the medium than the phase velocity of light? This is the case where a particle of matter "appears" to go faster than the speed of light through a medium. (Cerenkov radiation)

[@deadlyshizzno]

There's a great video on the PBS Be Smart channel about this exact subject

[@krwada]

@@deadlyshizzno Cerenkov radiation?

[@deadlyshizzno]

@@krwada yup. I think it's titled something along the lines of "when matter moves faster than light"

[@jelliebird37]

I was thinking the same thing about Arvin: he fills in the key aspects of physics that would otherwise make physics impossible for people to understand.

[@tedarcher9120]

It's a wrong explanation though

[@CaravaggioRoma]

this is an amazing explanation of the difference between phase velocity and group velocity!

[@CaravaggioRoma]

@@parapsia em waves "lose" time by being absorbed and re-emitted in all directions. these causes distructive and constructive interference,... which is a slower process overall for the final "winner" to emerge wrt a straight ray of light. so i expect that, the denser the material, the slower the light.

[@StephanBuchin]

​@@CaravaggioRoma- According to the interesting Fermilab channel video "Why does light slow down in water", absorptions and emissions is not the right explanation and his arguments are clear and solid 🙂

[@tristanbedard643]

@@CaravaggioRoma what I’m wondering is how the phase light remains the same but the group velocity is slower and why it affects only the observer and not the actual speed

[@hudsonurruttia]

I was so happy to see this video from you. I've been thinking about this since I finished physics II in my previous semester

[@apple1231230]

we have ways to "slow down" light to very slow speeds. this makes me wonder if you could surround spacecraft in a similar such material that would super distort the incoming light in a kind of camouflage. alien tech no doubt, but an interesting thought

[@MA-hd4or]

I really love the simplicity in this channel. It helps me comprehend very confusing topics. Keep it up...

[@ArvinAsh]

Thank you. Happy to hear that!

[@ferventheat]

Yet simplicity can lead to error, misrepresentation and you come away with an inaccurate understanding of the concepts presented and those needed to understand how 'light slows down and speeds up.'

[@tborabta]

Your explanation was absolutely mind-blowing! Introducing quantum mechanics alongside these phenomena has left me astounded. Thank you immensely!

[@philosophiabme]

Great video! I always wondered about this but never thought to look into it until now. It always bugged me to say that “light slows down” for some reason.

[@bnjm8868]

Thank you Arvid for your research and hard work. You never disappoint. This was very informative with eye-opening details. Great video, indeed! 😃

[@ryancormack6934]

Fascinating video. I've wondered about this question. Well done!

[@fredcunningham9753]

Thank you Arvin, you are the man. Your videos are super digestible and help me understand physics. I love it

[@ArvinAsh]

Thank you. Glad you like them!

[@onebylandtwoifbysearunifby5475]

Great explanations and visuals, as per your usual efforts. Clear and clean descriptions.

[@9126786]

Wow. Dear Arvin Ash, thanks for sharing this beautiful video. I avidly follow almost all of your videos. They sum up are such a great learning. 🎉

[@ArvinAsh]

Thank you. Glad you find them useful.

[@meesalikeu]

thanks doc ash i always wondered about this. i actually get it both classically and quantum because of your clear, concise and i should add not trivially, patient lol presentation for us.

[@emergentform1188]

Fantastic stuff, I've always wondered about this, Hooray Arvin much love!

[@MxGrr]

This video is so good. Great explanation, master Arvin.

[@whitehorse1959]

Thank you for adding the quantum explanation at the end. It makes more sense to me. 😎👍🏼

[@Tigrayspells-]

if you understand it well, you explain it easily....what a great man !!! ARVIN🙏

[@feelingzhakkaas]

Absolutely wonderful video with simple explaining

[@ultravioletdream]

So happy to to discover you Arvin :-) Keep up the awesome work you're doing.

[@ArvinAsh]

Thank you. Welcome aboard!

[@user-pu9qe1nn2r]

Finally someone who explains in such a leymann way...thanks 2 u sir...As the story goes, I asked the same question to my teacher & he deliberately confused the whole thing ending up with nothing... In the next video, plz make the video on "WHY EVERYTHING, Even LIGHT follows CAUSALITY, that it's not The Speed of Light but The Speed of CAUSALITY???" PLZ MAKE A VIDEO ON THIS(I SAW A VIDEO OF PBS BUT BUT DIDN'T CATCH UP)

[@chelseastarr9405]

Great explanation! I wish I had seen this when I took a guided wave optics class in university!

[@HqBlays]

Im a 14 years old kid and I love your vids, specially the ones about quantumn mechanics and the QFT, keep the good work!

[@jonathanreynolds2625]

The number of nights I have stayed awake trying to figure out what scientists have meant by this has been immense. Thank you for helping me better understand this.

[@jamesraymond1158]

Excellent, as usual. At first I was puzzled by "Or Does It?" It seems that in one sense, it doesn't slow down, it's just that the interference the light induces makes it appear to slow down. They never taught me that in optics class in the 1960s.

[@FunkyDexter]

They didn't teach you that because it's wrong... Group velocity is only different if the phase velocity of the two interfering waves is different. As you can see at 7:30, Arvin sneakily made the two waves move at different speeds to get the result he wanted. So the incident wave goes at c, but the stimulated wave doesn't? Makes no sense. The explanation is as simple as "e0 and mu0 are not those of a vacuum anymore, hence the speed of light is different". Just look up the definition of refraction index: it's the ratio between c and the phase velocity inside the medium. Not group velocity.

[@axtxt]

yup arvins explanation is wrong

[@alexalekos]

thanks for that vid arvin, I was wondering this exact thing last week

[@Bassotronics]

In the quantum mechanical sense, the ability for light to take all possible paths while going the speed it does is actually frightening.

[@daber6948]

Actually, every quantum particle has this property and by taking integral over all paths while using only classical mechanics you can derive quantum mechanics from the base. That approach is fascinating

[@jgunther3398]

@@7thD_JAILBREAK108 i think you mean instantaneous, not spontaneous

[@7thD_JAILBREAK108]

@@jgunther3398 indeed , am not English that is why, yeah I really meant instantaneous

[@anywallsocket]

@@7thD_JAILBREAK108even instantaneous is misleading, it implies light is not affected by the properties of spacetime between emission and adsorption.

[@anywallsocket]

@@daber6948you can derive CM from QM using Feynman’s path integral formulation up to Lagrangian dynamics, not the other way around.

[@AbhishekKumar-vc8yi]

Hi...I have a doubt. Why doesn't the photon do the same(following all possible path) in vacuum. Why is this superposition of paths happening only in a medium?

[@edwardjenner1381]

Great question and observation. In fact it does. All quantum interactions behave as if they are a superposition of all possible paths. But the other paths all cancel out in the limit to infinity. Even with other objects in space (stars, galaxies etc..), in the limit of infinite space, they cancel out. Plus anything beyond a certain distance is going to have limited affect on that summation. However, in a medium that is not infinite, they do not cancel, hence the result.

[@FunkyDexter]

Because the explanation is wrong, on multiple accounts. 1. The speed of light is not constant in general relativity, because gravitational fields are not inertial frames of reference. Locally observers still measure the same speed of light, but across larger distances they might see the speed change (it actually can assume ANY value depending on reference frame). GR supplanted the postulate "speed is constant" with "light travels on null geodesics". 2. Photons only exist as part of field-matter interaction. The EM field itself can take on any value of energy. Just think about the field generated by an accelerating charge, it forms a continuous spectrum depending on acceleration, which is not quantized. 3. There is a deep issue with his representation of the primary wave and the stimulated wave to show group and phase velocity (around the 8:00 minute mark): he's constantly talking about photons, but what he's showing here are two continuous waves interfering. Notice though how the resulting interference is comprised of wave packets (again, result of matter-field interaction). The description of phase velocity and group velocity makes sense if we're talking about continuous waves, but not if we're talking about quantized energy exchanges. I don't think he explains this clearly enough at the end of the video, where he just talks about "quantum nature" without explaining how the difference arises. 3.5 Another important issue with the group velocity picture is that it only appears when the two interfering waves travel at different speeds. He kind of swept it under the rug first showing the two waves traveling at the same speed with a phase difference at 7:20, then he sneakily made them move at different speeds at 7:30. So even if the incident waves were to travel at c even inside the medium, the stimulated wave would not. So which is it? If you look up the definition of refractive index, you find that it is the ratio between c (speed of light) and the PHASE velocity (not group velocity). I.e., the speed of light is different. 4. If we want to use the photon description, we need to use another reasoning. The real reason photons are quantized entitities is not that they contain a minimum amount of energy (well, they do, but this is a result of another property): they have quantized angular momentum, hbar. This is a direct result of emission from matter: the difference between adjacent levels of the quantum number of angular momentum in atoms (ml) is exactly hbar. Since the "wavepacket" is restricted by moving at the speed of light in a vacuum and the angular momentum must always be hbar, the result is the famous relationship E=hf. Note that f can take on ANY value. 5. In a semi-classical way, we can assign an "effective radius" of the photon to account for the angular momentum (according to L=mvr, with m being the effective mass of the photon E/c^2). The angular momentum applies a torque on charged matter in a medium, which in turn acts to reduce the angular momentum by an amount hbar/n^2, where n is the refractive index. The effective radius though is the same as in free space: the only way for this to happen to conserve energy is for c to be smaller. On this point I reference the papers by Loudon, Padgett et al., and Mansuripur. 6. Related to the above, Maxwell equations are very clear: c changes because mu0 and e0 are not those of a vacuum anymore.

[@edwardjenner1381]

@@FunkyDexter Which university did you go to to learn all this?

[@FunkyDexter]

@@edwardjenner1381 I have a master in chemistry. I had various courses dealing quantum mechanics. Needless to say, the onthological foundations were extremely lacking, which is why I researched these topics by myself.

[@Rationalific]

Incredibly fascinating! Thank you for not dumbing things down for us! You went two levels over what most would teach as fact. Most say that light slows down in a medium. Then, we learned the classical reason why light only appears to slow down. Finally, we learned the actual quantum mechanical reason. And although these things are hard to fathom, I appreciate your clear explanations with useful illustrations! By the way, I may be totally off base here, but seeing as how it seems that with interactions, everything that can happen does happen, I wonder if this can have some relationship with the "Many Worlds" hypothesis... It's easier to see how these worlds could recombine in this case, but the seemingly random nature of a single particle in the double-slit experiment makes it kind of seem like in that case, the worlds don't combine again and we just see one result, and the other worlds (not in other dimensions of space or time, but in other dimensions of probability) see all of the other results (one in each dimension) as we see ours... I wonder if there is anything to that...

[@cloudpoint0]

He skipped "Signal Velocity".

[@suyapajimenez516]

Thank Arvin for your great effort you put in educate the commons😂. I have to play the video several times to understand ( I was in the business of medicine, now retired) but I did it. By now I’m used to the weirdness of the quantum world but always surprises me. 🙏🏻❤️

[@ArvinAsh]

My pleasure. Glad it was helpful!

[@christianfaust5141]

Absolutly great, when I learned during my studies in electro optics that the phase velocity is given by c but the group velocity is slower than c in the medium I had always the problem to understand why we have the occurrence of the group velocity. It's the interference between incoming wave and the wave created by stimulated electrons with slightly different frequency. The quantum mechanic explanation is really weirder but quantum mechanics is about probability and what we see is the most likely path of all possible paths.

[@mattmcmillan3573]

Great video! Thanks Arvin!

[@jesusbermudez6775]

Thank you for these explanations of the velocity of light and why it slows down in a medium.

[@rezenpm]

Mind blowing and enlightening. Thank you for giving me a better understanding how nature works.

[@borahsilver478]

I love the intro music to your videos. Is there a full version I could download, listen to at the gym. "Can I do 30 minutes on the treadmill? The answer to that is coming up right now!" lol.

[@dr.gayfirstlookmri]

Wow! Excellent video yet again. Thank you for enlightening us on this super interesting topic.

[@ArvinAsh]

Correct, both are incomplete. But, QM explains many more phenomena, is extremely accurate in making predictions, and is very well documented. Even Einstein felt that GR, which uses geometry to explain gravity, was a stopgap explanation.

[@dvalentino7492]

Thank you this is been keeping me up at night

[@shaunmitchell2069]

Thank you very much I finally understand the contradiction in stars redshifting.

[@001firebrand]

I've been wondering about this issue since I was a 2nd term student. Thanks again, Dr. Ash!

[@The757packerfan]

As others have pointed out, Maxwell's equation seems to point to the opposite. You say the speed of light doesn't change. But you also gave us the equation c = sqrt( 1 / permeability * permittivity ). Don't the permeability and permittivity change from vacuum to air to glass to water? So doesn't that mean c (speed of light) changes?

[@PewrityLab]

You are correct. The phase velocity changes as well, just like 3b1b has shown in the recent optics series

[@KaushikAdhikari]

This is extremely informative ❤

[@ZohaibKhan-mr7uy]

Excellent explanation with good animations. Sixty symbol did an excellent video on light slowing down in medium. In which he is talking that when photon interact with medium it become a polatron which has mass as it leaves the medium polatron becomes photon again and move with same speed

[@ArvinAsh]

Yes, that is another way to explain it, but it is a bit more controversial.

[@ZohaibKhan-mr7uy]

Sir, I am if possible please do a video on the spin of particles. Why fermions has half integer spin and boson has an integer spin explain their symmetry. Thanks

[@ZohaibKhan-mr7uy]

Sir, if possible please do a video on the spin of particles. Why fermions has half integer spin and boson has an integer spin explain their symmetry. Thanks

[@josephbenson6301]

Good video... I've always found this basic topic a bit puzzling because, without adding energy, I always wondered how a photon slowed down, but then sped back up. That aside... Something you said in here clashed with another something I recently saw about the universe being a quantum object. That then made me wonder... If the universe is a quantum object, at the moment of creation (however that happens to work), would there have also been created a universe with every other possible come nation of qualities? (You know, one where up is down and light travels strictly at 55mph. And a similar one, where it's 56mph.)

[@leniterfortis4832]

I was JUST asking myself this a few days ago. Thank you for the explanation.

[@joachimkeinert3202]

Great exlanation. For more insight you can read Feynman's Lectures chapter 31. He explains how dispersion and refraction can be inferred from the constant velocity of photons (c) and the oscillating electrons of the atoms.

[@joeblow6865]

Arvin, I love your content!!!

[@InShadowsLinger]

I just can’t wrap my head around how we get from “math describes it this way” to “that is what actually happens”. As in math has to calculate every possible path, therefore light actually takes every possible path.

[@anoopkrish44]

Interesting and Useful

[@dravikumar4401]

Sir, really your explanation skills are amazing. But I am having a doubt that, in 6:37 you have told that EM waves are produced in electrons by its own but if that so electrons emit EM Waves without absorbing light??

[@ArvinAsh]

Yes, any moving electric charge will emit photons.

[@dravikumar4401]

Is that mean electric charge carries photons??

[@romeolupascu920]

Somehow the quantum explanation made more sense to me than the classical one... great video, thank you Arvin.

[@SantoshKumar-py4er]

Brilliant Explanation, as always !😊

[@matthewwilliams2128]

So what velocity of light do I see if I am moving relative to the medium it is going through? If light moves through water at 0.75C and I'm moving through it at v does the light appear to move at 0.75C still, or at 0.75C - v ? And how will the relatavistic length contraction of the water affect this?

[@dennistucker1153]

Most excellent video. I really like this one.

[@user-ss3hg5sr5h]

Your videos are great! Thank you very much

[@ArvinAsh]

thank you. Glad you like them!

[@user-ss3hg5sr5h]

@@ArvinAsh 🌷

[@yellowrosetv]

I love this channel ❤️❤️❤️❤️ been subbed for long long time 😍

[@Zeero3846]

You should follow up with an explanation of superluminal group velocities and negative group velocities.

[@rururu5877]

Thank you very much, i 've juste seen the entire video and i think that you look awesome !

[@rururu5877]

Coooll

[@timrwilson1]

Thanks! Keep up the good work.

[@ArvinAsh]

Thanks so much!

[@agressivepandit]

sir, information given by you is jist awesome #respect

[@deadlyshizzno]

this one definitely gave me a major a-ha moment! So cool that the superposition is what causes the perceived difference in speed

[@mmotsenbocker]

Can we define a photon as a single indivisible vibration that is created or destroyed (but not altered) by adding its energy to mass (bundle of standing waves?) or emerging from same as a traveling wave? Feynman taught that things such as electrons constantly absorb and emit such photon vibrations many many times as they approach and interact with other things.............................. Second, I would love to see photon polarization discussed since so many people are confused in that area/ On the one hand + spin and - spin photons are circularly polarized respectively but on the other hand isnt spin per se merely the direction and not the magnitude of the angular momemtum of the spin? The ambiguities I see might be explained by photons having mixtures of circular and linear polarizations.

[@ScienceAsylum]

Great video Arvin! You explained the difference between group and phase velocity well 👍

[@bili4591]

hey 👍, please continue doing accurate video like everytime

[@astr0nox]

Wait, if you say that the originating light waves do not slow down but the sum of those waves with the induced waves create a wave we perceive is slower, doesn't that mean that if I have a coil of one lightsecond of fibre optic cable with both ends next to each other, I should see light one end instantly and not one second later if I shine light at the other end?

[@DrDeuteron]

There are several video on this topic, and I think they pretty much just cause confusion. Classically, one can use the linear "in-media" Maxwell's equations (the ones with E, B, D, H), and it's clear the propagation of the wave is slowed down in media. The atomic electrons do not vibrate at all frequencies under the influence of the external electric field (that would make it impossible to match boundary conditions); rather, the entire atomic orbital is polarized (a dipole moment is induced) at the frequency of the background EM wave. This presents a bound charge that slows propagation down. The statement that a changing electric field induces a magnetic field (and vice versa) which leads to a self-propagating wave is useful, but technically incorrect. All fields are only sourced by charges, currents, and their time derivative on the past light cone. (See Jefimenko's equations). In this view, there is interference between the distant original source, and locally induced polarization that slows the propagation of the wave front. Saying that this a group vs phase velocity thing does not help. For instance, in a dispersion-less medium with index of refraction n, both the group and phase velocity are c/n. Dispersion only occurs if the index of refraction depends on frequency. phase velocity is w/k, and group velocity is dw/dk, which is a whole different video. Water waves are classic for this, as they differ by a factor of 2. Considering the classical EM wave as a bunch of photons is too advanced (see: Glauber State)...the number of photons isn't even well defined, thanks to usual quantum mechanics things. Now you can do one photon at a time (maybe a Fock State?), and I have: you need to account for propagation delay if you want to get the timing right, so as far as I'm concerned: it slows down. And if you want to understand the Feynman path integral formulation, and why it works somewhat intuitively with light, start with Fermat's principle of Least Time (with an understanding of Lagrangian and "Action") and then ready Feynman's short and brilliant book: QED, The Strange Theory of Light and Matter.

[@76097]

My other question is about two black holes merged and we can detect the gravitational wave use LIGO. Now based on GR, if anything falls into a black hole, observers from outside the black hole will see that object goes slower and slower while it approaches the EH, the light it emitts will be more and more red shifted, until we can't detect it any more. We are going to see it frozen near the EH. How can we even detect the gravitational wave of two black hole merged? Should they be seems slower and slower but NEVER will pennitraing the EH to us? Thanks!

[@micaiasoliveira6337]

Hey, Arvin, could you please tell me if in the quantum explanation for refraction the individual photon speed remains 299 792 458 m / s?

[@ArvinAsh]

Theoretically yes. Photons and all other massless particles cannot travel at any other speed but the maximum. They would not exist otherwise.

[@micaiasoliveira6337]

@@ArvinAsh Thanks

[@akildoktoru]

At time 3:21 we see an animation for propagation of light waves. It is also said that this propagation is induced according to Maxwell's equations 3 and 4. But in above animation, it is not clear to me what animated electric field change induces what animated magnetic field change and vice versa. Starting with time equals zero, can you please explain what parts of this animation induces what parts of this animation along time?

[@snehaltembhurnikar3314]

such an incredible explanation

[@fazalkarim3997]

If it is possible please arrange a lecture like this to explain the superluminal propagation of light.

[@ChannelSRL1]

Why does the group velocity only occur within the medium -- what happens to the EM waves of the affected electrons before exiting the medium? Why is the refraction consistent rather than random and why does it proceed out of the medium parallel to incoming line? Why isn't the light we "see" a different color owing to the increased wavelength between the perceived group points?

[@HadiLq]

Thank you for sharing these great videos with us, and I don't blame you for sharing the confidence that we know how it works, because we really don't. Let me ask you two questions to make it clear that we don't know what's going on in the medium. As scientists we should be okay to say "we don't know". - In the quantum picture that you mentioned, what is the assumption that differentiates the result of the path integral calculation in a medium, like glass, that just have effect on the group velocity, which preserves the direction of propagation, and the scattering of light in that medium, which doesn't preserve the direction of propagation? - if the path integral story is the correct description of this phenomenon then glass should be heated up much more than we observe, because the acceleration of those charges would face resistance, which will end up creating a lot of heat.

[@edwardjenner1381]

"Let me ask you two questions to make it clear that we don't know what's going on in the medium" Like you said, it is OK to say you don't know. However, you should not confuse 'you' don't know with 'science doesn't have a predictive explanation'. Those questions in no way 'make it clear WE don't know what is going on' . But I'll you a question based on your second question. How much should the glass be heated up by the path integral (feel free to choose your light source power and size of refactor).

[@jazznik2]

I previously saw another video (dont remember whose video it was) that explained this a little differently. IIRC the other video said that when light passes thru a medium of matter, the matter also has waves that combine w the light waves and these combined waves move slower than the speed of light. This video seems to explain this in more detail. I dont think the other video mentioned induced light waves. And it didnt mention the quantum mechanical explanation, which I dont really understand anyway.

[@Corvaire]

Well done as usual Arvin. ;O)-

[@tiwariarti76]

Bro bouta have his vsauce arc 💀

[@billcad15]

Not convinced that vibrating electrons in media like glass emit photons per se. Have seen some physicists refer to emission of "polaritons", which add to photons (thru superposition) some inertial mass/energy to slow photons down. It's photons themselves which "jiggle" electrons slightly. Polaritons are a property of electrons that get left behind when photons exit. What do you think about this way of describing the speed changes that are observed in different media?

[@MrOP66]

The animation of propagating electric and magnetic field 3:24, is it really like that? At the node points both magnetic field and electric field are 0, so where is the energy stored? If, however, you offset the sine waves by 90 deg, you would always induction taking place, and a way for energy to remain constant.

[@Blaizyr]

Does this mean that if each individual photon moves at the same speed regardless of the medium, yet the collective electromagnetic wave of photons has different parameters, is it solely due to the variation in path, which can be described by the probability distribution of path selection?

[@aguma2067]

Un vídeo fantástico 👏👏👏

[@sihoonoh9021]

something I don't understand. according to the animation, it looks like it is just the apparent summed up wave defined by the movement of the shape formed by constructive and destructive interference that is traveling slower. but the original wave which is one of the waves that compose the summed up wave is traveling at c. So wouldn't this mean that the information that the light was shone travels at c even in glass? As long as the original wave only takes up even a tiny proportion of the total light intensity.

[@OrdenJust]

Excellent video. It proved to be a fertile source for contemplation. In particular, I remembered a statement of Dirac's: “each photon then interferes only with itself. Interference between different photons never occurs.” So I wondered. Do photons interfere with other photons or not? Was Dirac right? Was Dirac wrong? Or was Dirac accurate, but only within the limitations of the experiments that he had in mind? Mandel and Magyar's experiment of 1963 might have the answer. Or might just lead to more questions.

[@elmolewis9123]

Excellent presentation.

[@billcox6791]

The classical picture given makes sense in the steady state it reaches, but it also seems to imply that the leading edge of the wave will reach the other side of the medium at the speed of light in a vacuum as the individual waves are still propagating at that speed. Does this need atomic or quantum effects to resolve, or am I missing something simpler?

[@ArvinAsh]

Yes, leading edge would be at c. Atomic effect are quantum.

[@taylorwestmore4664]

​@@ArvinAshCould you do a video exploring quantum electrodynamic effects? Situations where light destructively interferes but non-trivial quantum effects become apparent are very confusing but it does make some sense that the quantum potentials would continue to propagate at C, while the EM force fields appear to propagate at a slower phase velocity due to superposition. Examples like the Aharanov-Bohm effect and the the dynamic Anapole mode are situations where the destructive interference of EM results in quantum potentials being non-zero in regions of space where Electric and Magnetic fields are classically zero. There is still debate about the physical significance of the potentials in the formulation of QED, even since Feynman famously elevated the status of the magnetic vector potential and electric scalar potential in his lectures to "more real" or more fundamental than the EM field. Thank you!

[@jonathandawson3091]

5:30 is the best fourth wall break that I've ever seen in a video.

[@IntraFinesse]

Arvin, You would get a ton of search hits if you add the tag "How does light travel slower than". many people think the Photons have slowed below C and search results don't yield web pages with as good a description as you have made.

[@ArvinAsh]

Good suggestion! Thank you.

[@trigonzobob]

Is it me or is the induced wave traveling slower than the original wave in the animation? What about a material causes the group velocity to be different than other materials?

[@GaryFerrao]

This is the correct explanation of our present scientific knowledge. Better than some other videos that mistakenly say that light travels longer because of randomly scattering off atoms.

[@Alann103]

Interessantíssimo!!! Interessantíssimo!!! Interessantíssimo!!!

[@Paul-ty1bv]

The elections, protons, and atoms induced to emit photons because of the motion of the passing electromagnetic wave: from where do the particles obtain the energy to emit a photon? Is the kinetic energy converted to photons? Does the incident light wave lose any energy as it gives it up to these particles to move them? Loved the video btw.

[@salec7592]

But, since the c figures in phenomena which apparently are not in any way connected to electric or magnetic permeability (each obviously being derived from the other by using c) of vacuum, like gravitational waves, there has to be some more fundamental cause of its universality, constance and magnitude. Can you tell us more about that?

[@jorriffhdhtrsegg]

What i dont understand is how in the double slit type experiments, numerous glass filters are involved from polarisers and beam splitters and how do we know this doesn't interfere, if so much interference is happening?

[@kayakutah]

Great explanation.

[@maddoghel]

According to your explanation, Dr Ash: One of the "possible paths" -but not so probable- isn't for the photon to go back to its entry point into the medium? How's that manifested? Could you explain, please?

[@ArvinAsh]

Yes, it could take all possible paths. But you have to remember that there are billions of photons typically in visible light. The likelihood of all the photons taking that path is so astronomically small that it would not happen. But if you sent just one photon through the glass, then yes, this could happen, and would be detected.

[@maddoghel]

@@ArvinAsh Thank you, for your time to reply and your answer, sir!

[@studyguru2656]

😂😂Sir in your video at t=6.01 the speed of light is shown 299,292,458 m/s but the actual speed of light is 299,792,458 m/s. By the way thanks for this knowledgeable video❤❤

[@dimitriosfromgreece4227]

AMAZING VIDEO 💗🙏💗🙏💗🙏

[@muhammadmohaiminulislam7189]

Similar thing is called "chotpoti" in Bangladesh. Except for the sauce which is made of tamarind and no ghee is used. Loaded with corriander leaves(much more than you used here), green chilies, crushed "fuchkas" and the spice as garnish.