I think 1 thing is missing here: the actual speed of light in the usual sense is not the phase velocity, but the group velocity at the front of the wave packet (which determines when the first part of the wave packet hits something). The light wave didn't exist for all time, but the 1st part of the light wave reach the electron at some time, and only after that the electron can also oscillate. So even in the case of x-rays, the group velocity would still be the vacuum speed of light, but the refraction is determined by the phase velocity, which is why x-rays are refracted away from the vertical.

Why is it always the channels with fewer subscribers that have the best explanations? You could not have explained this better!

That's one of the best videos on the internet! I really appreciate your work, because we don't have this kind of depth in high school's physics (speaking from Brazil here), so it's really great to understand it properly!

I just wanted to take some time to point out how useful your videos have been not just to my to my knowledge of physics but also my perspective on learning. It is easy to believe a subject or topic is boring and not worthy of learning just because of who is presenting the material, without taking into account that everything can be made interesting with passion and love for something. Thank you @floatheadphysics and I hope you continue to make videos

Wow!!..blown away ...thankx mahesh...u r the best educator ever i seen on youtube...no one explained that way,they spoon feeded on this platform about light

You sir, are the utmost representative of intuition and simplicity, just like Feynman. I'm pretty sure if you were to be involved in research some 30 years later (teaching higher students), humanity would be thankful to your contributions! :)

Best and fastest and most intuitive.explanation I've found on internet about the ligth transmission through a material. Before your explanation I had seen unclear explanations from some other well known physics youtubers, and I was a little clueless. Thanks for your clarity and simplicity. I really appreciate it, since it's not easy.....

I truly appreciate your content that you put on youtube,,,you are one of the best teacher delivering quality content,,,please do post more videos including dispersion of light ,i would love to have it... Thank you

I like how you actually wave your hand when you get into the hand waving explanation.

Wow. I found your videos just recently. I have an engineering degree from a long time ago and I wanted to understand special relativity. I have some textbooks. I’ve watched a lot of different videos. Your videos are outstanding. Thank you so much!

This is a brilliant explanation of something I had never even considered. It's almost 60 years ago since I was in college studying Microwave theory, which covered a lot of the same ground - wave phases adding or subtracting - but it was on a space commuications course (NASA), and we didn't go into optics. I knew from general interest how light refracts in glass, and I had always accepted on faith that it is because it slows down in media denser than free space, but the idea that it doesn't slow down - it just takes longer to traverse a glass block of fixed dimensions - is a relativistic thing - and Relativity is not intuitive. But I didn't know that X-rays refract away from the normal. I can't say that it is all plain and clear to me now, but I can, at least, follow the argument. So refraction in glass, then, is dependent on the wavelength of the wave being refracted. I can't see if it would be directly proportional (I suspect not), but I'm sure there is a fairly simple mathematical expression that relates wavelength to angle of refraction. I've just subscribed and become the 565th person to give it the old thumbs-up, and I'll be looking for your other videos. Clear explanations of scientific concepts are few and far apart! I've only seen this one video of yours, but I put you in the same bracket as Feinman - and, incidentally, Eintein, whose book "Relativity" (ISBN 0-517-02530-2) is also a brilliantly simple, non-mathematical explanation of a subject so complex that when he first proposed his ideas on Special Relativity, only a few other scientists of his day could come to grips with it. I'd like to ask some questions, if anybody can answer them ... 1) Are there transparent materials other than glass where the natural frequency of their molecules can be determined (eg water, or perhaps dense gasses), that obey the same refraction formula, based on frequency of the incoming wave, and their own natural molecular frequency? 2) Has the time for light to traverse a block of glass been experimentally determined? Does it back up this lecture mathematically?

I love how u used topics that are familiar with everyone to describe this crazy phenomenon .Keep up the good work.

Do they give Nobel prizes for youtube videos? Because you're blowing my mind over and over again...

I have struggled with these concepts for years . Thanks to your explanations, physics has now become so much more interesting, and easy to understand. I have watched so many videos trying to understand quantum physics but believe me you are the best . Keep up the good work.

This explanation and insight was great. Thank you.

I suspect this is as good an understanding as possible without introducing the maths. Very well done.

I love how this topic has been covered by you, 3blue1brown, and Looking Glass Universe, and you each have different ways of approaching how to explain this complex phenomena. Thanks for doing this work and bringing such passion to every video!

After watching your video its my time to showoff my knowledge in physics to my friends....i am sure that i blown their minds exactly like what you did to me 🤯🚶🚶💯

Fantastic video! Best explanation I've seen!

I can't believe how good this explanation is. Amazing

Excellent work! Thank you!🌈

Bro I love watching your videos. Please continue making more!

Superb way of explanation... wonderful

Super.... superbbbb brother. I gotta found your Chanel and watching almost all the videos up here.

Very good. The video from Fermilab got me halfway there but I was still unsatisfied because he didn't go into how the delay of the phase of the forced wave can vary depending on frequency. The x-ray thing too was surprising. I am now satisfied to the extent that I want - I don't feel like slogging through papers because I'm way too lazy for that! Thank you much!

Not sure that the X-ray wave is pushed forward, that would be the wave influencing the electron before it gets there. Tather, the X-ray wave is so small that the influencing wave is pulled back beyond the crest of the wave, resulting in the phase of the previous wave appearing in the next wave. (if that made sense)

Great And Great Explanation!! Great Work!! the besttt explanation! Specially without maths, Bestt!

I would say that the more intuitive way of thinking of the xray situation isn't that it pulls the wave forward, but rather it seems to because it delays it beyond a half wavelength so it appears to move the peak forward through a kind of wagon wheel effect

bro physics with you is art ! No video explained why light is delayed in a convincing way except this one, you made the difference by going into the details with is great, the details really make ALL THE DIFFERENCE, it's like to show someone how a home was built brick by brick instead of showing him how the home was built floor by floor, the brick by brick explanation method is the best and actually you are the only one I found explaining why the light slows "brick by brick" ( with great details). Keep going like this bro you are doing awesome work, and know that it's DETAILS that always make the difference in explanation ( explaining step by step without jumping steps). BTW I also think the same way as you with intuition, I found myself asking to me the same question you asked to yourself and you thanked the same way as I tried to do !

Interesting point of view. I'll need to think about it more, I'm not on board w/it, especially the xray theory but understand the thought. In my mind with xrays being much higher freq, the electron will only be 180 out of phase every so often, not for each wave. The video post; The Primer Fields Part 3 may interest you, maybe check out. The xray reaction has issues in the Primer Fields too. Excellent subject, look forward to having some extra time to think about it. Nice post. Look forward to reading the comments.

thank you so much for great explanation

Thanks again for another great video. My 40 year old college degree is in EE, so I spent a lot of time thinking about sine wave addition. Your explanation made a lot of sense to me. Maybe those who don't have that background would think that one explanation is as good as the other. But without your explanation, I would have continued to be confused. None the less you did not address the change in angle, just the change in apparant speed of the light wave peaks. Would a spherical representation of the light leaving the electron help? I don't know. Thanks again for excellent clear videos!

I really liked your explanation.

Boom! I know this feeling my brother! It is the joy when you understand something so complicated. It is the beauty of Science!

Very nice and complete explanation

I always had this doubt in my mind that why is it said that light slows down or sppeds up because according to the Special Theory of Relativity, the speed of light is a constant, irrespective of the medium it travels through and then I would think what happens during refraction. I even asked my teachers but didn't got an answer. But now I know the answer just because of you! I really thank you soo much for this video and all the other amazing video that you have uploaded (also on Khan Academy) I just had one question, why does the light bend? The light wave just got shifted backwards due to the resultant new wave but why does it bend away or towards the normal?

Thanks a lot for this explanation ❤

Amazing explanation ! 👌👌🌹

Superb explanation ever

It makes sense how the light gets pulled back but why does it bend? You should something like light is going forward but an effect of coming back shows. Like when a wheel moves fast forward and it looks like its moving back or in a fan sometimes or a broken tv screen. Sorry it might be dumb question but wanted to learn more

I still don't understand how this resulting phase change of the light causes it to bend 🤔 Also, shouldn't the electric field of the electron also affect light behind it once it starts oscillating?

You could have put diagonal electron waves in the picture to show how the lagging works or draw some kind of triangle to show the length difference. But that was the only part I had to skip to because I didn't understand it immediately. And the fact that I understood the rest what you said very well shows that nonetheless your explanation was really good. Great video👍

Great video!

Thank you for the wonderful explanation. Does the wavelength of visible light actually shrink in the glass ? Because this then would increase the frequency and create energy right?

God has graced you with a very intutive mind keep it up

Thank You sir I was really in doubt about this matter that does light acctually slows down ur vedio has helped me a lot to understand it. 😊😊😊😊😊😊

Than you for this cool video. It blew my mind also as you showed that with the conservation of energy you can guess the position of the secondary wave. AMAZING :) But here 2 additional informations: 1. As the electron moves up and down it emitts the Photon with a phase of 90° later according to the displacement. So the blue wave (secondary wave) is in the minimum 90° behind the yellow wave (primary wave) up to 270° behind the yellow wave. 2. Optics books never explain the situation, what is happening during the time as the first wave hits the glas. It always describes a steady-state situation: The light wave already passed through the glas, as we see it here in you explanation. So the claim of beeing faster then light ist mathematicaly correct, but the information actually already passed through the glas cause the whole glas is already filled with a light wave. So no information is moving faster then light. Never saw a mathematical explanation what is actually happening as the first wave hits the glas.

Interesting. But how does this explain bending? And why metals reflect light so well?

Amazing!!!

Hi. Could you share some of the resources /reading material you used while researching this topic. Thanks. The video is insightful.

I like the fact that you reply to all the comments. Btw your class 10th science video is helping me so much.😁

There's a little problem in 16:40 time mark. It shows like the X-ray passing through the glass can reach the right side of the screen sooner than the X-ray passing through air. A more reasonable explanation is that the X-ray phase is delayed by more than a half wave per wavelength of propagation, creating an illusion that it was sped up less than a half wave. Hence, actually the X-ray propagates through glass even slower than ultraviolet light, which is slower than visible light.

Wonderful explanation sir ❤❤🔥🔥

Ohh Wow What a great explanation ever Thank U thank u thank u very much I am very very happy now. I am having a confusion on it ❤❤❤😁😁 keep going.. 🙌👍👍👍

I love your video! Thank you for this work! However, it opens new questions: 1) If the wavelength in glass becomes shorter then it should become shorter and shorter for thicker glass. And as far as I know, this is not what happens in reality. so, what is going on? 2) I was taught that photons deliver their energy to electrons only in packest of energies, shifting the electrons into different energy levels around the nuclide. So, how can the photos deliver just a bit amount of energy to the electrons? Thanks again

Excellent video, from far the best one I found about the subject thank you really much, but please bro I still have a very small question, at 12:00 when we look at the green beam as you said it is behind the yellow one (its peak), but how can we interpret the small piece of green beam that is ahead of the green peak ( at the extreme right of the screen), what is it ? because when the yellow peak will reach the receptor the green peak won't reach the receptor but instead of the peak it will be this small piece at the extreme right, so do the yellow and the green beam arrive at the same time but in a different phase (like yellow will arrive at its maximum and green at its minimum) ? Please bro I have been searching for 2 hours but I didn't find...

Perhaps this could be extended to explain why glass is good at reducing UV rays? Well done

Well explained....in terms of wave but how about photon, particle perspective ????

Amazing 😍

Great explanation thank you! To clarify, the light still reaches destination at time d/c right? As in I can’t cheat and a laser going through a glass rod wouldn’t actually win a race with a laser in vacuum - information can’t get to me in under or over d/c if I’m looking at the source through a glass rod.

this was just perfect

That is very insightfull. So between visibile light and x-rays, is there a frequency where light goes through the material completely unchanged? As this will depend on the material in question, there would always be a frequency for which this material is perfectly invisibe, right?

Why does displacement take place though, lateral or longitudinal? That has always confused me.

So the phase shift of the induced wave depends on the light's frequency relative to medium's resonant frequency?

My own, admittedly, imperfect analogy is to think of crossing a chasm on a bridge made of swinging boards. The boards' swing frequency can not be changed and the runner's step frequency can't either. However, long legged runners swing their legs slowly and short legged runners swing their legs fast. So all runners have the same speed (lambda times f = c). A runner who swings his legs slowly may find himself hitting the boards when they are going back and so is always delayed. A fast swinging runner may hit the boards when they are swinging forward and, thus, crosses the chasm faster. It all depends on matching the leg swing frequency to the board swing frequency. If a runner's leg swing just matches the board swings he may end up not feeling the boards at all and his timeing to cross the bridge would be the same as with non-swinging boards (refractivity index of that bridge would be 1 for his frequency). If the board swings are just as fast as the leg swings, the runner may end up going nowhere and it could be said that he has been absorbed; as in red light not passing through blue glass. There's a reason why swings are always used to explain resonance frequency.

Would wish you would use a pop-filter in front of your mic. The popping sound can become distracting especially listening with headphones. Very interesting videos. Love them.

actually you are my best KZfaqr

Thank you. I really like your explanations. I think in this case the induced wave gets delayed so much that it appears in front of the next peak. Anyway, could you explain how destructive interference works? You know, if you shine the laser light on an object, split it in two and delay one half by half period. Then the common understanding is that you will not see the light on the object. How is that possible from the conservation of energy point of view??

Yep, fantastic.

Accurate interpretation, very nice. But how does this approach explain the refraction (bending)?

Question: If visible light bends downward through a medium and X-rays bend upward, is there a frequency that can travel unaffected through the medium?

Ok sir, i get why light seems to slow in glass, but why does it actually bend? Like is it because it has to travel in longer time but with swme speed?

21:07 can we just say that the electron in the mean position is in Direct influence of that electric field and the electrons above and below will get to the direct influence of electric field but after some tiny time delay.... 🤔

Hey! and what about the light emitted in the same plane but to the left? it should also be considered for the conservation of energy, right? btw excellent video. Very well explained!

I have a question about 10:12 where you say that it violates coservation of energy because energy is destroyed are we sure some of the energy dosent just transfer into momentum for the electron? Because if its shaking up and down once the photon hits it once it past wouldnt it just keep going after once the last peak stops interacting? the way i see it the osscilating fields cancel out this momentum allowing the electron to basically be trapped in the photon but one it leaves wouldnt it leave an impression in the momentum sense there will be a poin where the ossciallation stops? wouldnt this allow for a photon with weaker magnitude?

Sir i think this was all about the speed of wave which you explained beautifully and someone has a good knowledge of interference can understand it easily but you did not explained why it bends? Or did I missed something 👀

At 14'07, I don't understand but it seems that there is more periods in the signal travelling in the medium, like if the frequency have increased (it looks like phase modulation in signal transmission), is that possible ? Why can we explain the change of direction that does not appear in the figure ? Thanks for all :)

Mind blowing..

Your explanation in general is correct and I congratulate you being one of the few giving the correct explanation but IMO not entirely. The laser visible light wavelength is too large to make the electrons inside the atoms (glass is a dielectric it has no free electrons) in the glass to oscillate similar to the photoelectric effect) . It is not a dynamic EM wave interference effect but simply the quantum electrostatic scalar matter field of the glass material compresses the wavelength of the light inside the glass. In the case now of the X-Rays there indeed we have oscillation of the glass electrons and light waves interference effect that changes the phase velocity (not group velocity c) of light inside glass which becomes effectively faster that c value (refraction index n

Excellent video, although I have a question, what happens with the waves "we see"? I mean, to measure anything, there must be light that comes out of the thing being measured, and approaches the thing that measures. For example, how do we know that the light is not doing something else, and it's the light that goes towards the measurement apparatus (e.g., our eyes) what "implements" the effect?

Sir thank you for the clear explanation of the speed of light in medium But i have doubt sir ,how the light bend in travel to rarer medium to denser medium please explain sir

From where does electron get energy to oscillate? If electron is getting energy from em wave ,doesn't it effect original wave?

In the double slit we go from EM waves to one photon at a time, and using the wave-function of the photon getting the same (interference) results So what if I send one photon through the medium? Same results perhaps by treating it quantum mechanically?

OMG brilliant!

that was really cool

When passing light, does the temperature of the glass remained completely unchanged or does it heat up? If the glass shows an increase in temperature, wouldn't that indicate absorbation and dissaption of energy from the light source?

I'm still catching up and this is perfect

You never actually talked about refraction. We can see that "slowed" visible bends toward the normal effectively shortening it's path in the medium (which I assume exactly compensates for the "slowdown". While the "speed-up" X-ray bends away from the normal effectively increasing the length of its path, again cancelling the effect of the "speed-up" So that the transit time across the thickness of the medium is that same as if the medium wasn't there in both cases. Right?

consider the analogy of exciting one tuning fork with another tuning fork of a different natural frequency. Energy conservation constraints will require the excited tuning fork to ring at the same frequency of the exciting fork, but somewhat "quadrature" in phase. Amplitude of the ringing of the excited tuning fork will depend on the difference between the natural frequencies of the two forks.

Need a full video explaining why light goes slower and x-rays go faster in glass... Atomic level explanation.. waiting.......

As far as i know, when light hits the glass, there is a momentum loss, which results with a phase shift, that we call refraction.. am i wrong? If the way you tell is correct, the composite waves’ amplitude should have changed? Why didnt it? Or u just omitted the diff bc the electron generated one is so small in amplitude? Besides fourier analysys can be of help.

14:10 - if the wavelength shrinks, the frequency increases, so each photon has more energy. This looks like it violates energy conservation, no? Unless the light intensity there is less than before and these two effects balance out? Is this what the explanation is here? 16:28 - also with xrays, this would imply causal action can now travel between events between which it couldnt without the help of xrays+glass. How do we solve this paradox? My explanation was that the very first part of the wave travels as if no interference happens, and the "speed up effect" only happens to the parts of the wave that follow. Because the electron's EM wave isnt actually ahead (in phase) of the incident wave, but is so far behind that it looks like its ahead. This would mean the very tip of the incident wave remains unaffected? No clue if this is right so please correct me The reason i said the electron EM wave cannot actually be ahead of the incident is because electrons take some time to accelerate once the electric potential at their position is changed by the incident wave. 23:04 - if the further-away electrons are not in the right positions wouldn't that mean that the blue wave isnt in the exact correct phase difference to the incident wave, hence there wont be equal amounts of constrictive and destructive interference. This suggests that things like the density of the glass (or any property that affects the arrangement of electrons) messes with energy conservation. Apparently the quantum version of this model is more precise in the details but i havent looked at it and i probably won't understand it. Just one last thing - Ive heard people say the group velocity is lower in glass but the phase velocity is constant. Is this true? If the light isnt actually slowing down (or speeding up), but the pattern of the wave is pushed back, then information should still flow slower. Imagine an experiment with a detector behind a block of glass, measuring the speed of information in glass. With visible light, because the electrons' EM waves lag behind, the tip of the incident wave will make it to the detector as if there wasn't glass, right? But what about after a while - wont information flow slower? For x-rays the tip also takes the expected time as if there wasnt glass. But for information in the rest of the incident wave - wont it make it to the detector faster than if the glass wasnt there? That's what the animation in the video suggests.

How the direction of light changes , is it due to phase change in medium

Does this mean that the first photon that goes through a medium will go through the other side at the time as if it travelled with the speed of light?

At what frequency would there be no lead or lag? (In glass, I guess.)

as an electronic engineer, I would call it retarded phase. In electronics, information can be transmititted by modulating the phase. I found this video very interesting.

Mahesh: You might be loosing me at this point. Me: Mahesh, my friend, that's simply untrue. Anyway great content, even though I thought I might end up blaming you for taking away time from my gaming session, I just can't at this point. Yeah, I'd be super interested to understand dispersion of light using this concept, would help solidify the concept itself too.

Then, how do you explain regracive index as the ratio of the velocities

A very good explanation, but there's one question. Why is the wavelength in a medium not the same as the wavelength in a vacuum? Since all waves propagate at the speed of light in a vacuum, shouldn't the wavelength remain unchanged after superposition?

so the exit wave from glass is forever changed from the input wave?