Why does light slow down in glass?

Let's explore the age old question. Why does light slow down when it travels from vacuum to any other medium?

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[docta]

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

[rfvtgbzhn]

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.

[solus]

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

[jonahansen]

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

[monukr]

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

[victorgoncalvessoares]

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!

[ozachar]

This is the absolute best and most rigorous explanation to be found anywhere on the web, or in any physics classroom (and I have a PhD in physics). Fantastic presentation.

[darshan]

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! :)

[binitasingh]

After 3 years I finally got my answer, I have been asking this question to my teachers for ages but they just said ' THE LIGHT SLOWS DOWN 🤓' and if I asked why then some of them said ' LIGHT COLLIDES WITH GLASS'S ATOM AND LOSES SPEED 🤓 ' theni get frustrated and asked them WHAT ABOUT EINSTEIN, HE SAID THAT SPEED OF LIGHT (ie C ) IS CONSTANT IN ALL SITUATIONS, then they just ignored me.😅
Thanks Mahesh bhaiya
( Btw I'm still in school 😢 )

[varsha_]

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 🤯🚶🚶💯

[blazereflx]

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

[mikefochtman]

Great discussion. When you asked how the light affects the electron, my first thought was it should be shifted by 90 degree. The electrostatic force of the light wave on the electron is maximum when the wave is at a peak/ valley. But that means the electron is 'accelerating' maximum at that moment. With simple harmonic oscillators, the acceleration is max when it pauses and reverses, i.e. when it momentarily stops. And the 'wave' formed when it's not moving is zero and it would be maximum when the electron is moving fastest (just as the original electrostatic force is crossing zero).

That would make the wave formed by the oscillating electron, 90 degrees out of phase with the wave that forces the oscillation. Of course, electron orbitals are probably much more complex than simple harmonic motion, but something at least to think about.

[rockapedra]

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!

[martifingers]

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

[stevecarson]

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!

[Achrononmaster]

@19:20 you might not know it, but this point here is the most critical piece of the puzzle by far. Light is *_not_* a wave, the EM theory is a "hydrodynamics" of photons, applicable when you have a lot of photons. If you try to follow a single incident photon (with a gedanken Heisenberg defeating microscope!) it may not even emerge, it could just get absorbed, or Compton scatter, or small chance for whatever other scattering cross sections. Your wave effect explanation really is beautiful, but some minor inconsistencies some listeners might note could be resolved by knowing that it is only a statistical mechanical explanation. EM fields are fictions. Albeit very useful.

[rafaelcalderon]

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

[Khalid-Ibn-Al-Walid]

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 !

[tablettorrensabellan]

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

[DownhillAllTheWay]

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?