@@stevenutter3614 I appreciate your comment that appreciated the prolonged thread of appreciation, as I realized how I assured the right spelling too :)
@youareonthetube1 Have I just seen a different video from yours? I did not hear the word "lie" in it. He does not talk about conspiracies here. All he is saying that some people in an effort to explain things to lay people (and themselves) are wrong or incomplete. Has your physics teacher when he talked about gravity explained curved space to you? I do not think so. In making things easier some people get too far with simplification and others have not understood the matter themselves that they are trying to explain. Science is about falsifying wrong hypotheses and explanations and that is exactly what this guy has done. Please tell me in what publications in scientific magazines you have spotted fraud and misinformation and proove to us what is wrong with it. Just saying that lies are getting spread without showing what and where is dishonest.
I still can't understand how this bending of light is any different from the gravitational lensing of light by massive objects (like black holes). That's due to gravity curving space time around it and light following the curved path. However, doesn't it hold true in this case too? May be glass being denser than air ( analogous to a galaxy being denser than vacuum), leads the space time around and with in the glass to bend a little bit (way smaller in magnitude though) and as a result we see the light through glass's denser gravity? (Similar to the Eddington's experiment to prove Einstein's theory of general relativity by seeing a star's apparent and not the real position in the sky due to its light bent by sun during a total eclipse. Similar to how we see the position of a coin placed at the bottom of a water jar to be slightly raised due to bent rays? And Einstein definitely used Maxwell's equations in his general relativity. So, couldn't this all be connected and we're understanding refraction wrong all this while? I don't know.
Vivek density and gravity are not the same thing; an extremely dense object will have identical gravitational force as a larger more diffuse object (as long as the mass/energy of the two objects is identical). One way to think of this is if the Earth is compressed to a size of roughly a 1cm diameter. Obviously it is extremely dense at this size, but it’s total mass/energy is the same. The space time curvature at the distance from 1cm Earth to the moon will be identical as the curvature from normal Earth to the moon despite the difference in density. Thus the moons path around 1cm earth will be identical to its current path around the Earth.
I don't think I'm alone in saying that your videos //can't// be too long - I would happily watch you present a documentary. Thank you for your work, Doc!
Watch some of Leonard Susskind's lectures where he will give you a glimpse of how stuff really is if you have an appetite for more. You won't know how to do the math but you can follow his arguments as he goes through them and will come out spellbound.
Ditto! I'd honestly rather watch your videos than Neil deGrasse Tyson's or Leonard Susskind's! (Maybe because I'm more interested in quantum physics than astronomy, for the former, and because you work from scripts, for the latter.) Maybe not more than Sean Carroll, though. Oh, but if you could do a feature-length documentary _with_ Sean Carroll, that would be incredible!
Oh my God, finally i got the answer, i asked this question to my teachers, colleagues and family many a times about what actually happens at the atomic scale, they either ignore or explain those faulty explanations. Now i can die in peace Thanks you Dr Don Lincoln, 🙏🙏🙏 Love From India🇮🇳🇮🇳🇮🇳
Maybe mark deep dive videos similarly and only create them according to what interests viewers or creator. Keep the short ones as it keeps the channel alive and more input from viewers. Maybe a weekly deep dive to start?
Nice when the presenter is also the thinker and loves the topic. This channel is our favorite, correcting many misconceptions and wrong / incomplete teachings. Dr. Lincoln clearly and succinctly explains.
Yet, there is something wrong with his explanation! It simple don't work for TE polarized waves... So, he is replacing good explanations with a wrong one
He never said there was anything wrong with those principles, he only claimed and demonstrated why they are not the causal explanation of the phenomena.
I was searching for this answer since i have developed my intrest in physics. i have searched a lot but was never able to find a proper answer. Thank you so much. i am satisfied now. I can now die in peace
@@alephii by TE do you mean transverse electric, any light with a transverse electric field? As in the electric field oscillates orthogonally to the direction of energy transfer? As in any monochromatic light in a homogeneous linear dielectric? I get you're trying to sound smart but you literally just tried to use the video's example as an example that wasn't explained by the video.
@@lachlanbunney6545 still, TE light also bends. The physics can't be different, I am not trying to be smart, just pointing out that if there is a flaw in the explanation then it is probably wrong!
@@lachlanbunney6545 By the way, a transverse electric field in TE doesn't mean that electric field oscillates orthogonal to the wave propagation, because that is obvious in the case you mentioned, but TE refers to the electric field transverse to the incidence plane, the plane that contains the incident and reflected light... The explanation is incomplete and probably wrong, would be much more precise to use the phase continuity in the interface...
+Lakshya Goel You really arent going to get far in mathematics or physics if you keep asking why questions - the universe works the way it works, be glad it gave you a chance to experience it.
@@NinuRenee lol wtf! The whole point of science is to explain things, curiousity is what leads to discoveries. You are pretty much an animal in forest who doesn't know anything if you just say "it's the way it is, no fucking reasons for it to be" shut the fuck up if you can't understand things, don't demotivate people who actually want to find logic and explaination for physical phenomena....
@@affinix887 Maxwell's equations tell us "how" electricity and magnetism behave, they don't tell us "why" the equations are what they are. Science finds out and describes "how" nature works; it doesn't and cannot tell us "why".
0:29 OMG my comment in a TH-cam video for the first time ever. I'm so happy for some reason. But Thank You so much +Fermilab for clearing up our confusion.
Absolutely awesome explanation! The marching soldiers explanation always bothered me, and (regarding speed of light in matter) I could tell the scattering or absorption/emission explanations had to be false because the direction of light would change. It is incredibly refreshing to finally have straight answers to these fundamental questions! Thank you so much!!
I love that you know and clearly identify the distinction between resultant description and physical causation, as well as the distinction between a mathematic description and its physical interpretation. It is too easy to stop at fermat's principle as the cause or to stop at the mathematical derivation and just say "the math says so." Very well done!
It is surprising that even people who are into science communication on TH-cam end up teaching wrong stuff and we need channels like this to spread the truth. Please consider my request on making a video on The Principle of Least Action!
I taught science & marked HSC physics over 34 years & although I have since retired from teaching; I learnt something that I didn't know. You are always learning & that is what makes life soooo inteesting!
34 years teaching science and marking hsc physics…and you never knew the answer to this question, nor in 34 years try to figure it out on your own in case someone asked?? What else don’t you know and never took the time to learn answers to basic questions?? YikeZzzzzzzz
Yes, I taught Science; geology, chemistry, biology & physics along with mathematics over the 34 years. You are obviously ignorant of a teachers work load to somehow think that you have time to find out & research every nuance of every subject that you teach. There was not a weekend that I was not marking assignments, tests or setting them. Apart from that also engaged in students welfare. Then you have a fool like you make the comment you make! @@AirborneAnt
These videos are really saving innumerable people from carrying on with the misconceptions....this way is totally so much effective to explain the misconceptions first and then the inconsistensies of them and then finally to explain the correct answers...a very few youtubers do so...i remember veritasium also said about the effectiveness of this method on the people...Thanks Dr.L
Methinks: "ha you obviously haven't heard of my analogy because it's simply fantas..." Dude: "number two, the marching soldiers anology" Methinks: "godamnit"
The one I remember being taught is the rolling barrel analogy. If you imagine a cylindrical barrel (more like an oil drum) rolling from pavement to grass at an angle, upon striking the grass first, one end will experience drag relative to the other and produce a torque similar to what Mr. Fermilab explained with the "rigid" lines of soldiers. It does replicate the geometry, but like Fermat's principle it provides no explanation as to why photons (which are often considered points or at least spherically symmetrical) would behave like rolling cylinders.
Here a Graduate Physicist. There is also another simple explanation for this: (As consequence of the conservation of momentum of photons) If in medium 1 the index of refraction is n1 and in medium 2 the index of refraction is n2 and for the definition of index of refractions and wavelength we have: n1 = c / v1 , v1 = λ1 * f n2 = c / v2 , v2 = λ2 * f Dividing the above equations we obtain: n2 / n1 = λ1 /λ2 So if n2 is higher than n1, the wavelength in medium 2 is smaller than in medium 1. Now because the photon momentum is: p = h / λ Applying the conservation of momentum of the incident and transmitted photon along the parallel line of medium separation: p1 = p2 → h/λ1 *sin(θ1) = h/λ2 *sin(θ2) → n1 *sin(θ1) = n2 *sin(θ2) This is the Snell Law!! So the light bending is just a consequence of the conservation of momentum of photons!
@@scottdc2105 "The conservation of energy only applies to anything with mass": Not true. Conservation of energy always applies . In the case of electromagnetic field (photons), conservation of energy is guaranteed by Poynting theorem: en.wikipedia.org/wiki/Poynting%27s_theorem Also note that my proof doesn't use conservation of energy but conservation of momentum. The conservation of momentum in electromagnetic field is guaranteed by the Maxwell stress tensor : en.wikipedia.org/wiki/Maxwell_stress_tensor
@@scottdc2105 A particle without mass can have energy and momentum. The classical formulas (p = m*v , E = 1/2 * m *v^2) are not valid for particles with speed near the speed or light or with the speed of light. Instead we need to use the quantum and relativistic formulas: p = h/λ , E^2 = (p*c)^2 + (m*c^2)^2
I'm so glad to see fermilab explain this so acutely! It's all about the capacitance of the glass to store the electric field of the light, counteracting it, and finally returning the stored energy of the light upon it's exit.
Sir , you are a LEGEND. This is the best explanation of refraction ever , in my graduation i learned this E perpendicular and E parallel but understood it today after 9 years, thanks for making such a video. Love from India🇮🇳🇮🇳🇮🇳
Thanks for clearing up a misconception that was taught to me that I've been passing down to my students. Why doesn't the parallel component have to decrease, I missed that.
Because both initial components are actually derived from boundary conditions for M and E part. So this is only valid at entrance point. Further on, the wave propagates straight through that medium and both components are affected by the medium and as long as the medium remains the same, it behaves like perpendicular entrance to that same medium. Otherwise the light would curve in that medium which is not the case.
Apparently the ‘epsolom’ (sorry don’t know spelling) is bigger in glass than in air. This effects the perpendicular component. (I’m not sure myself why this doesn’t affect the parallel component too. Apparently it’s just the perpendicular component 🧐) Because of this, the angle of the perpendicular component is different to that of the parallel component. It is made shorter This creates the angle we see when light his glass.
He didn't explain it. 12:22 He showed E field entering perpendicular to the surface, thus the electrons in the glass lined up horizontally to cancel it, so he 'cleverly' deduced that the perpendicular E component got reduced in the glass. If E field entered at an inclination (which should be when light entered at any random angle), then by same logic, electrons in the glass should line up in appropriate direction to cancel it, so both parallel and perpendicular E field components should change.
I think it's because the beam's vertical electric field is moving against the intrinsic electric field of the material, whereas the beam's horizontal electric field isn't. I'm only guessing though, and I'll defer to anyone who actually knows what they're talking about.
He did not show it. But said that we can get there after some calculus. The so called boundary conditions between two media are shown in em.geosci.xyz/content/maxwell1_fundamentals/interface_conditions/derivation.html It uses the Maxwell equations mentioned in the video, but in its integral form.
@@saspinski That's the math. It doesn't explain the physics. You would think the charges in the material would get polarized by the parallel component. The dielectric constant is also just an approximation, because it assumes the material is continuous.
To fully appreciate this result, you'd need to read the electromagnetic theory. The electromagnetic theory says that light is an electromagnetic wave, it is nothing but oscillating electric and magnetic fields. Electric fields and magnetic fields satisfy a set of 4 equations, 2 for electric fields and 2 for magnetic fields. The 4 equations are different for different media or materials. But at the boundary of two media, both equations of medium 1 and medium 2 must be satisfied, because the boundary belongs to both the media. So the electric field equations in one medium must be equal to the electric field equations in the second medium. This is what we call a boundary "constraint". The electric field is constrained to move in such a way that the boundary conditions or boundary constraints are satisfied. It turns out the boundary constraints you get on solving the equations is that the parallel component of the electric field must not change, and the perpendicular component must decrease. Please not that it is ok if you fully do not understand this, this is something that is taught in undergraduate physics course.
Thank you for this explanation, I was taught the soldier reasoning ( 1966) at school and it never seemed plausible to me. I have no formal science education except high school science but I am an avid follower of all science ( subscribed to the New Scientist magazine for nearly 50 years ) and now subscribe to quite a few science based channels on the internet. This is a great time to be alive !!!!
formolzinho a teacher once told me when I asked how long my paper should be his answer was that it should be like a skirt. Long enough to cover the subject yet short enough to still be interesting.
anyway I don't understand nothing because in genere physics today are made to don't understand nothing because instead to be something fun and relaxing is contrarity like a very long run of a deer in a forest. everything can be very clear if you want tolearn something but to learn something what is not so precious like a new form of energy bec ause this planet must clean herself from many forms of pollution. I heard light people don't knows what is it.
I have derived those two results about perpendicular and parallel components in my electromagnetic field theory class but never thought it had so much connection with light bending...i wish my professors were like you to tell the real stuff and not making me derive these results for mere marks in exam.
Yeah, that has always been a problem for me. I'm fine with learning equations and seeing that it works, but I want to know how it works, otherwise I'm not understanding a god damn thing. 'The sun shines' is not an explanation as to why it emits light. Might as well say god diddit and here's an equation that can calculate the angle when it hits glass.
Talking about radio waves: These can be bent and change polarization in electric fields - think antenna theori. So that should also be possible with light? Is there any examples of bending light or changing polarity shining it between charged plates?
This is the BEST explanation I've seen thus far!!! And I used to teach Physics for 20 odd years!!! And the fact that one can derive this result purely mathematically, using Maxwell's Equations, makes this a clear winner! This upload: *Why is light slower in glass? - Sixty Symbols* did a pretty good explanation using the idea of electromagnetic waves, but then it got bogged down with the chaotic and cumulative effects of the quantum electromagnetic nature of the wave's photons, interacting with the quantum electromagnetic effects of the atoms of the medium, as they too oscillate and produce such waves themselves as photons pass near them. Indeed, a new 'particle' to describe this phenomena: *the Polariton* was used in an effort to explain what might be happening in a quantum mechanical sense. I'm sure such effort have their merits, even though the maths involved must be INSANE so, this very simple approach in relative terms, using just the classical laws of Maxwell's Equations, for me makes this explanation the clear winner!!!
Did you know that first explanation was the inspiration for the short story "Story of your life" which was made into the movie "Arrival?" It was somehow explained tot he author that light must somehow KNOW what the shortest path would be when going through a medium change. That meant that light had already seen the future. I guess it was really Variational principle in general, but the light thing caught him first.
There are differing motives when explaining physics: 1. Describe what happens in a way people can remember WHAT happens. 2. Describe what happens in a way people can have a rudimentary understanding of WHY it happens. 3. Describe what happens in a way that only physics students can understand. I like that you took a middle ground between 2 and 3 in this video. Explaining physics to non-physics students is tricky business. :)
12:17 Isn't the arrangement of charges wrong? I mean if the glass particles were to generate an electric field in a direction opposite to the total electric field then the -ve ones must be above the +ve ones because only then we would get a field opposite to the total electric field.
I searched a lot for this question but no answer was satisfying me, but your explanation is amazing, I never ever even imagine that the knowledge of electromagnetism is behind the bending of light
after watch 2 videos, i think electric part of light is not enough to properly explain reflection think about it. reflection need parallel and perpendicular part of electric field to flipped 180 degree AND material generated electric field stronger after hit by light to make electric field flipped someone can correct me if im wrong or maybe have better explanation :D
I think reflection is explained by the fact that light is changing magnetic and electric field not just one or the other, the change in electric field gives rise to magentic and visa versa , in the reflection case it the electric field goes to zero as it is completely completely reflective material and hence the change in electric is negative which produces magetic field in the opposite direction which then induces electric field in opposite direction hence reverse the perpendicular component of the field while not affecting the parallel one and effectively reversing the direction of propagation in the perpendicular direction.
What a spectacular video, the explanation, the graphics, the editing just perfect. If they taught science like this is schools, everyone would go for science
I am a new teacher covering the topic of refraction for the first time. ALL of my resources talk about the "Marching soldiers" theory. I thought I was going crazy. I ketp asking "But WHY?!" I really appreciate this explanation.
And yeah, almost forgot. This wasn't a long video. I really like good explanations. A good explanation deserves ALL the time needed for that. I wouldn't mind longer videos in the future if it is required. I'm subscribing cause I love physics and this channel makes me love physics even more.
In the animation, the electric charges of the molecules in the glass moves/reorient themselves according to the direction of the light, but light travels through my window from many directions at the same time. So please explain how each light beam can bend when it transitions from air to glass, consistent with the equtions. One would think the charges of the molecules in the glass gets confused by the bombardment of light travelling through it from various directions? BTW, I love your videos - sooo much better that what you stumble upon on the web.
What a hard question! I'm delighted that you have asked it. May I say it's all about the supremacy of geometry over physical mater. Any model that includes light waves or rays, and in particular the directions of these waves and rays is wrong by definition. Every such a model is by necessity a 3D structure displayed in an instantaneous space. It doesn't meter if these models were kinetically animated, they are still only the sequences of frozen 3D frames. BTW, the eternity could be the three dimensional only. To be able to answer your question one have to realize that there is no underlying reality of pictorial, i.e. geometrical, models and they should be looked at as if they were eternal objects themselves. However, there is no such thing as the 3D eternal model of light, or of electromagnetic extension in general. Light does have a peculiar direction which is always toward the observing, receipting, point. If you are interested in ... Respect
I've learned this derivation at university and I remember that a key part in the derivation was that it is impossible for the electric and magnetic fields to exhibit discontinuities except for the case of a superconductor. Could you explain why the fields need to be continuous and why they are allowed discontinuities in superconductors?
I wasted a lot of time trying to understand explanations in undergraduate physics books. Often it just was too contrived. Thanks to channels like this, one gets real insights.
At 12:05, shouldn't the charges move around in the opposite way? Positive must be downwards and negative upwards. BTW thank you so much, watching your videos really help me understand Physics better.
Wow! I love how this channel approaches seemingly complex physics, with easy-to-understand explanations. The real mechanism for the bending of light is really interesting. But it creates several other questions: 1) As molecular rearrangements differ between different facets of a crystal, does this mean that different facets of crystals have different refractive indices due to different electric field environments? 2) Does this mechanism also explain total internal reflection? Or to be more precise, how does it explain total internal reflection?
Very nice video. The example shown is for TM polarized light. Given that the boundary conditions are quite different for E and B, it'd be interesting to explain why we obtain the exactly same deviation for TE polarized light :)
I am really astonished as a physics teacher.Exellent explanation.i am highly satisfied.For the past 20 years I simply explained by mathematical expressions.Now prof explained with electric field concepts is seriously amazing
Thank you for the lovely explanation. I have a couple of questions for you and would be grateful for your thoughts: 1. Why doesn't the parallel component polarize the charge/substrate molecules? I mean what's happening physically that prevents that, not mathematically? 2. If I pump in external charge to a substrate using something like a Van de Graaff generator, would it change the permittivity and hence the ref. index of the substrate?
You have to wait... someone is in the back room inventing long math equations. The show will start in fifteen minutes and you will be dazed and confused. A great show !!!
After seeing your explanation many other videos also appeared in my feed telling the reason why light bends? Now I understand why it was more important to know the wrong explanations!😀 Thanks!!💐
So Electric field due to matter in glass makes electric field of light less. Does that mean we can actually bend light with strong enough electric or magnetic field?
Despite what some equations will try to tell you the answer is NO - there are zero examples of being able to bend light with either electric or magnetic fields. The ONLY example involves a medium such as oil, in which case it is the polarisation of the medium that changes, not the light itself. Don't believe equations, they can explain anything, instead look for real world examples - there are none.
12:47 I assume the re-arrangement of the electrical charge takes some time. So what happens with the "first" photon hitting the glass. Is it going in a straight line because of the not re-arrangent electrical charge? Or is the re-arrangement itself already influencing that photon since the re-arrangment itself needs energy to happen? How can that influence happen over a distance if the light is going by c? Greetings
@@TheAbizgreat10 as far as i know that actual light, or that specific photon will be absorbed by the atom, but now the atom is excited meaning it has more energy wont be stable at that higher energy level, so it will re-emit the photon, but that is not physically the same photon but just the copied version of the original, so it looks like light comes trough glass, but its just the effect of it what comes trough. photon itself in theory moves in a circular shape, which has its own wave like motion, but when light comes trough glass its a different kinda wave, its more classical physics kinda wave effect, that one atom reemits a new photon, than that photon will travel with 300.000 km/s but only until its captured by the nearest atom, then again atom is excited but not stable, reemits photon again, so the light travels 300.000km/s all the time when its actually travelling, but absorbing and re emitting takes time, and that will make it look like that light is slower now, but its just a kinda trick this wave effect is now similar to the sound wave, where the sound is not actually gong trough stuff, its just the effect of it appears the other end of lets say the glass door, so u can hear a person talking to u and u can see them trough a glass door, but how the photon is actually moving in vacuum is a different story , probably no one knows how s that happening on its own, cause no one knows what the electromagnetic field is physically
Here he has treated light in a "classical" manner, like a wave. The whole "photon" (particle) interpretation and analysis does not apply in this case. To understand it in terms of the direction a photon is moving we would have to use quantum mechanics equations and principles like the uncertainty of position-momentum.
YT should reveal the average duration and average number of times a video is viewed, backed up and o/w actively engaged. Thank you, FermiLab! Everything IS Physics!!
Ok, this is a good classical explanation, but let's go back to Fermat's principle for a moment. In particular I'm thinking of the explanation given in Feynman's little book "QED" (which I recommend to everyone's attention). Now it's true that Fermat's principle (i.e. of "least time"), as stated in the video, doesn't actually provide an "explanation". But when combined with the path-integral of the wave-function of the light Fermat's principle simply falls out of the physics. (Now, I want to clear that my respect for Dr Lincoln is absolute, and I'm perfectly willing to accept that I'm missing something here. But i'm not sure what that is.)
Least action of path integrals is a real and important thing, but it's pretty non-intuitive for a 12 minute video, especially where a lot is debunking common explanations.
Still, the Fermat's principle on its own doesn't explain anything. Feynman's path integrals are really not a common knowledge that would be used in "dumbing down" the underlying causes for light bending on an interface. Being a _part_ of one of the possible solutions doesn't make the Fermat's principle a correct explanation.
@@Tomas.Malina : > Being a part of one of the possible solutions doesn't make > the Fermat's principle a correct explanation. Um.. of course it doesn't Which is precisely why I made this point in my comment.
I was wondering about TE waves in the video explanation... For TE waves there is no perpendicular electric field yet the wave still bend... Did I miss something or the explanation is incomplete and fails for different wave polarization?
Thanks! Great video! Reminds me of what one of my professor told me long ago. Classical geometric optics is just Maxwell's equations + matching boundary conditions
Lincoln's Huygens explanation misrepresents the theory. It's terrible, and it's because his wavelets originate only for each new wavefront impinging upon the interface. This is not Huygens idea though. Huygens ' wavelets originate at every point along the interface for each wavefront. The entire point is that all of Lincoln's extra wavefronts don't exist at all according to Huygens, because those are decoherent.
It is because frequency of oscillating electric field is different for different colors, so you get different values of perpendicular and parallel oscillations, causing that slight change in direction for different colors (hence separating them)
I would just add that the plane of the exit surface of the prism is a factor. Leaving the glass through a surface angled from the entrance surface, the mix of photons that entered, exits on different vectors organized by frequency. With an exit plane parallel to entrance, like a windowpane, they take different angles through, but individually reverse by the same amount on exit, so they all end up back on the same path (as undifferentiated white light).
Because refractive index or the epsilon depends on the wavelength. For different color of light we have different epsilon so the extent of their bending will also be different. That's why they separate from each other!
Thank you very much. That makes a lot of sense. The way physics taught at school is so dramatic. They could have taught relativity and Maxwell's equation to us first and build other knowledge on it, but they rather create the suspense and mess up our understanding in high school, and only give us the big reveal in the very late stage of university or even higher education. I appreciate the setting, but it took way too long, that just renders any good story incomprehensible.
Great job, I love this channel, knowledge on the highest level that I can fully trust. I've been looking for an answer for this question after watching last video but nothing convinced me. Now I know why :) P.S. I also like your sense of humor, and I'm the person that prefers logical explanations than equations :P
Perfect explanation!!! Thanks for this video. Maxwell equations give exactly the needed framework to explain most of the phenomena. These completed with the boundary equations and field conservation principles are a really elegant way to solve complex problems.
Ima go out on a limb and say this video is timeless. It will inform, teach, and enter people's minds year after year upon one man's discovery of the hidden gem in the the galaxy of social media. Forever is forever.
unless the glass has isotropic properties, also parallel electric field should induce charge orientation on the horizontal axis, thus the parallel field should be dumped by the inducted field... but this violates continuity of the electric field on the interface... so where am I wrong?
I really like these videos. The two that deal with light's speed in water and how it bends were very enlightening to me. I'd also like to see one on how light reflects from a surface at the same level as the other two.
damn. This was so much needed. I had been searching for this answer. I almost had given up on this. Before this, I had an impression that it follows the marching soldier analogy with the fixed line, the only difference was I assumed that fixes line to be electric field but finally got the correct explanation.
Thank you for the detailed explanation on the physical nature of what Epsilon means. I'm curious to know how the Tand Delta (or loss tangent) relates to this? Specifically I would like to know how does the energy in the field gets converted into heat within die glass. Thank you in advance.
@@joshuascholar3220 Yeah. The multiple solutions mentioned in this video actually *are* physically real, if they don't cancel out with other such solutions. If there's physical structure with details on the same order of magnitude of the wavelength of the light, you get diffraction grating, thanks to all those multiple solutions cancelling out differently. That's why optical disks are rainbow-y. Oily puddles get the same effect from how thin the oil layer is on top of the water. Man that's a good book. I need to read it again.
When I found out that a beam of light travels towards the normal when moving into a material of higher density, I came up with the marching soldiers explanation on my own. I don't understand the physics to fully grasp the real reason light bends, but I'm so thankful for this explanation!
Dear Fermilab. This second video does not really tell the story either. This is just another way of deriving Snell’s Law. The crucial point here is why only the normal component of the E field is affected by the permittivity. The “real” reason for the behaviour of light in a medium in this context is that light represents a force that in turn induces dipoles in the medium. This gives rise to a London force that always is attractive. Any component of the attraction other than the normal cancels out by an opposite force on the other side. Think of concentric circles on the surface around the focus that each result in an attraction. The larger the radius, each circle have more slanted induced dipoles. This is also the reason that the reflection results in polarization effects (Brewster). It is illustrative to imagine an object on a transverse wave. In an idealized setting the object will move only up and down - there is no sideways motion. Likewise, the London force resulting from the incident light will only affect the normal component (of the EM field in this case).
Hi, I found your comment the other day and it seems like the answer I'm looking for but I've been having a hard time getting at what exactly is happening. I think what you're saying is, look at an atom at the top "layer" of glass that's been polarized by the light and turned into a dipole. This dipole will polarize its neighbors. The neighbors on the same layer should apply a force to stretch that dipole out more in the transverse direction, but their forces cancel. The neighbors on the second layer stretch the dipole in the normal direction, and this force cannot be cancelled since there is no layer above the first. This almost seems to work out, but if the transverse forces cancel out and the dipole remains polarizes in the transverse direction, that component of the electric field would also decrease
That was a great explanation. Now I would like to know, why some materials let light pass, some absorb light or some reflect light. What is the physics behind THAT?
Matter is made up of atoms, atoms have electrons in specific orbit(al)s around the nuclei, each orbit has a specific and fixed energy gap with other orbits. Suppose there are two orbits with energy gap of E, and you supply exactly E amount of energy to an electron in the lower orbit, it will take it and will jump into the higher orbit, if you don't give that specific energy E, little less or little more and the electron will not accept it! Now, lights come in different wavelengths and hence energies, some lights will have that exact amount E and that specific atom (of that matter) will accept it (absorb) and will get excited... you send light of slightly different energy and it will just pass through (refract). By the way, all that refract, they reflect too, in general, they scatter light....
@@atanunath Hi atanu, I am confused with the absorption thing. Is absorption always related to transition? I ran into some thing about absorption to x rays as a function of the x ray energy. It says the absorption decreases with the x ray energy gets higher until it reaches a specific energy under which the inner electron transition happens and abrupt increase in absorption occurs. I knew that absorption happens alongside the transition of electrons, but is all absorption linked to electron transition? What happens during the decreasing region I mentioned above. English is not my native language. Hopefully I pass my idea clearly.
Thanks, very nice. question: What if the light is polarized // to the interface (E does not have a perpendicular component)? Then B will have to adjust I suppose?
If I understand your question correctly, if the light is polarized such that the E is field is parallel to the interface then then beam is entering the glass perpendicular to the glass (an angle of 0 with the normal), and the beam will move into the glass and NOT change direction. The light will slow down (see the video he refers to at the beginning of this video).
Mr. Swaney, not in general. In EM waves, E is perpendicular to B and also to propagation direction. there are two basic possibilities for E. The video uses so-called s-polarized EM waves. In s-polarized waves, B is // to interface and E is perpendicular to B. In p-polarized waves, E is // to interface and B is not. You may just search for "polarization of light" for explanation. for p-polarized light, I guess one may invocke the continuity of Maxwell Eq. for B at the interface and get the same explanation for the bending.
@@Hiking-guide-and-scenery, ah, I think I understand your question now. What if the E field is directed into/out of the screen as we watch the video. Good question! As he said at ~9:08, he was only going to focus on the electric field. But it seems even then your question still applies.
@ Mr. Swaney, Yes that's it E into or out of page. well, I suppose he would change teh explanation from 9:33 and say "we keep only the 2 top equations..." But it is easier explain with E field as he says, because the B field would involve current loops~~
I like how this channel pays attention to the comments, and tries to clarify things.
I appreciate it!
I appreciate your comment appreciating the creator's appreciation of the comments.
Zachary Collier I appreciate your comment appreciating Flying Skyward's comment appreciating the creators appreciation of the comments.
I would really appreciate if he could explain to me how that explanation would work with a TE polarized wave (look for my other commentary).
@@stevenutter3614 I appreciate your comment that appreciated the prolonged thread of appreciation, as I realized how I assured the right spelling too :)
@youareonthetube1 Have I just seen a different video from yours? I did not hear the word "lie" in it. He does not talk about conspiracies here. All he is saying that some people in an effort to explain things to lay people (and themselves) are wrong or incomplete. Has your physics teacher when he talked about gravity explained curved space to you? I do not think so. In making things easier some people get too far with simplification and others have not understood the matter themselves that they are trying to explain. Science is about falsifying wrong hypotheses and explanations and that is exactly what this guy has done. Please tell me in what publications in scientific magazines you have spotted fraud and misinformation and proove to us what is wrong with it. Just saying that lies are getting spread without showing what and where is dishonest.
this is the answer I have been searching for the past 15 years (since my 9th grade). Thank you very much.
DotaLove I also asked this question in the 9th grade I didn’t get an answer until now. I am 70 years old.
So true they always told how it bends but never told why this happens
They were like you will learn this in higher standards
I still can't understand how this bending of light is any different from the gravitational lensing of light by massive objects (like black holes). That's due to gravity curving space time around it and light following the curved path. However, doesn't it hold true in this case too? May be glass being denser than air ( analogous to a galaxy being denser than vacuum), leads the space time around and with in the glass to bend a little bit (way smaller in magnitude though) and as a result we see the light through glass's denser gravity? (Similar to the Eddington's experiment to prove Einstein's theory of general relativity by seeing a star's apparent and not the real position in the sky due to its light bent by sun during a total eclipse. Similar to how we see the position of a coin placed at the bottom of a water jar to be slightly raised due to bent rays? And Einstein definitely used Maxwell's equations in his general relativity. So, couldn't this all be connected and we're understanding refraction wrong all this while? I don't know.
@@richriley5832 😱😱😱
Vivek density and gravity are not the same thing; an extremely dense object will have identical gravitational force as a larger more diffuse object (as long as the mass/energy of the two objects is identical). One way to think of this is if the Earth is compressed to a size of roughly a 1cm diameter. Obviously it is extremely dense at this size, but it’s total mass/energy is the same. The space time curvature at the distance from 1cm Earth to the moon will be identical as the curvature from normal Earth to the moon despite the difference in density. Thus the moons path around 1cm earth will be identical to its current path around the Earth.
I don't think I'm alone in saying that your videos //can't// be too long - I would happily watch you present a documentary.
Thank you for your work, Doc!
Watch some of Leonard Susskind's lectures where he will give you a glimpse of how stuff really is if you have an appetite for more. You won't know how to do the math but you can follow his arguments as he goes through them and will come out spellbound.
Ditto! I'd honestly rather watch your videos than Neil deGrasse Tyson's or Leonard Susskind's! (Maybe because I'm more interested in quantum physics than astronomy, for the former, and because you work from scripts, for the latter.) Maybe not more than Sean Carroll, though. Oh, but if you could do a feature-length documentary _with_ Sean Carroll, that would be incredible!
+1
yeah me to
I emailed him one time and he told me he has lecture you can find them at the Great courses plus website under Dr Don Lincoln.
Oh my God, finally i got the answer, i asked this question to my teachers, colleagues and family many a times about what actually happens at the atomic scale, they either ignore or explain those faulty explanations.
Now i can die in peace
Thanks you Dr Don Lincoln, 🙏🙏🙏
Love From India🇮🇳🇮🇳🇮🇳
Me too , some professors like HC Verma etc explained , but they were wrong too becoz they used the Huygens principle which is indeed impractical
Baat to sahi hai lekin hard core physics wala answer samkh me kuchh nahi aya .
Hi indi
Yahi to Indian Education system ki khasiyat hai
@@akanshavishwakarma272
?
I normally don't want to know the wrong answers.. but your style made it worthwhile...
I don't think this was a very long video. I'd love to see more 10+ minute videos on this channel that go into depth on a physics topic!
AGREED
Me too. I really prefer the long videos, not the short, with deep dives into the subject matter
same
Didn't even realise ten minutes passed.
Maybe mark deep dive videos similarly and only create them according to what interests viewers or creator. Keep the short ones as it keeps the channel alive and more input from viewers. Maybe a weekly deep dive to start?
Nice when the presenter is also the thinker and loves the topic. This channel is our favorite, correcting many misconceptions and wrong / incomplete teachings. Dr. Lincoln clearly and succinctly explains.
Yet, there is something wrong with his explanation! It simple don't work for TE polarized waves... So, he is replacing good explanations with a wrong one
He never said there was anything wrong with those principles, he only claimed and demonstrated why they are not the causal explanation of the phenomena.
This is my favorite channel for debunking every explanation I ever learned in my college physics classes.
What had you debunked?
Fermat's principle is right!
Not complet but right!
Finally, this has been driving me crazy for years. Literally years.
I was searching for this answer since i have developed my intrest in physics. i have searched a lot but was never able to find a proper answer. Thank you so much. i am satisfied now. I can now die in peace
No need to die. Live long and prosper 🙂.
@@deanschulze3129exactly 😊
This video already fueled my interest in Physics.
As a high schooler, I've waited years for someone to explain the correct reason of why light bends. Thank you so much. 😭
And are you sure this is the right answer? If you understood it well then tell me why TE polarized light also bends...
He's wrong Concave Earth
@@alephii by TE do you mean transverse electric, any light with a transverse electric field? As in the electric field oscillates orthogonally to the direction of energy transfer? As in any monochromatic light in a homogeneous linear dielectric? I get you're trying to sound smart but you literally just tried to use the video's example as an example that wasn't explained by the video.
@@lachlanbunney6545 still, TE light also bends. The physics can't be different, I am not trying to be smart, just pointing out that if there is a flaw in the explanation then it is probably wrong!
@@lachlanbunney6545 By the way, a transverse electric field in TE doesn't mean that electric field oscillates orthogonal to the wave propagation, because that is obvious in the case you mentioned, but TE refers to the electric field transverse to the incidence plane, the plane that contains the incident and reflected light... The explanation is incomplete and probably wrong, would be much more precise to use the phase continuity in the interface...
I love this channel because when I asked this from my teacher she simply it said,"its a law' and never explained........
Oh my, this is the worst way to teach anything!
+Lakshya Goel You really arent going to get far in mathematics or physics if you keep asking why questions - the universe works the way it works, be glad it gave you a chance to experience it.
@@NinuRenee lol wtf! The whole point of science is to explain things, curiousity is what leads to discoveries. You are pretty much an animal in forest who doesn't know anything if you just say "it's the way it is, no fucking reasons for it to be" shut the fuck up if you can't understand things, don't demotivate people who actually want to find logic and explaination for physical phenomena....
@@affinix887 Maxwell's equations tell us "how" electricity and magnetism behave, they don't tell us "why" the equations are what they are. Science finds out and describes "how" nature works; it doesn't and cannot tell us "why".
@@rclrd1 That's because the why question is meaningless.
0:29
OMG my comment in a TH-cam video for the first time ever.
I'm so happy for some reason.
But Thank You so much +Fermilab for clearing up our confusion.
Absolutely awesome explanation! The marching soldiers explanation always bothered me, and (regarding speed of light in matter) I could tell the scattering or absorption/emission explanations had to be false because the direction of light would change. It is incredibly refreshing to finally have straight answers to these fundamental questions! Thank you so much!!
I love that you know and clearly identify the distinction between resultant description and physical causation, as well as the distinction between a mathematic description and its physical interpretation. It is too easy to stop at fermat's principle as the cause or to stop at the mathematical derivation and just say "the math says so." Very well done!
this guy has the best ending line out of all science channels on youtube.
It is surprising that even people who are into science communication on TH-cam end up teaching wrong stuff and we need channels like this to spread the truth. Please consider my request on making a video on The Principle of Least Action!
we need this, please!
Yes!!
yes please!
+1
You might find Coopersmith's _The Lazy Universe_ helpful: www.goodreads.com/book/show/33570590-the-lazy-universe
I taught science & marked HSC physics over 34 years & although I have since retired from teaching; I learnt something that I didn't know. You are always learning & that is what makes life soooo inteesting!
34 years teaching science and marking hsc physics…and you never knew the answer to this question, nor in 34 years try to figure it out on your own in case someone asked??
What else don’t you know and never took the time to learn answers to basic questions??
YikeZzzzzzzz
Yes, I taught Science; geology, chemistry, biology & physics along with mathematics over the 34 years. You are obviously ignorant of a teachers work load to somehow think that you have time to find out & research every nuance of every subject that you teach. There was not a weekend that I was not marking assignments, tests or setting them. Apart from that also engaged in students welfare. Then you have a fool like you make the comment you make!
@@AirborneAnt
These videos are really saving innumerable people from carrying on with the misconceptions....this way is totally so much effective to explain the misconceptions first and then the inconsistensies of them and then finally to explain the correct answers...a very few youtubers do so...i remember veritasium also said about the effectiveness of this method on the people...Thanks Dr.L
Methinks: "ha you obviously haven't heard of my analogy because it's simply fantas..."
Dude: "number two, the marching soldiers anology"
Methinks: "godamnit"
Haha
The one I remember being taught is the rolling barrel analogy. If you imagine a cylindrical barrel (more like an oil drum) rolling from pavement to grass at an angle, upon striking the grass first, one end will experience drag relative to the other and produce a torque similar to what Mr. Fermilab explained with the "rigid" lines of soldiers. It does replicate the geometry, but like Fermat's principle it provides no explanation as to why photons (which are often considered points or at least spherically symmetrical) would behave like rolling cylinders.
Atleast you guys were taught analogies...😐😐 #fml
In our schools they usually make you memorize the formulas and don't even dare to create some stupid wrong analogy to explain things even for a while
Here a Graduate Physicist.
There is also another simple explanation for this:
(As consequence of the conservation of momentum of photons)
If in medium 1 the index of refraction is n1 and in medium 2 the index of refraction is n2 and for the definition of index of refractions and wavelength we have:
n1 = c / v1 , v1 = λ1 * f
n2 = c / v2 , v2 = λ2 * f
Dividing the above equations we obtain: n2 / n1 = λ1 /λ2
So if n2 is higher than n1, the wavelength in medium 2 is smaller than in medium 1.
Now because the photon momentum is: p = h / λ
Applying the conservation of momentum of the incident and transmitted photon along the parallel line of medium separation:
p1 = p2 → h/λ1 *sin(θ1) = h/λ2 *sin(θ2) → n1 *sin(θ1) = n2 *sin(θ2)
This is the Snell Law!!
So the light bending is just a consequence of the conservation of momentum of photons!
The conservation of energy only applies to anything with mass and nothing with mass can travel at the speed of light.
@@scottdc2105 "The conservation of energy only applies to anything with mass": Not true.
Conservation of energy always applies . In the case of electromagnetic field (photons), conservation of energy is guaranteed by Poynting theorem: en.wikipedia.org/wiki/Poynting%27s_theorem
Also note that my proof doesn't use conservation of energy but conservation of momentum. The conservation of momentum in electromagnetic field is guaranteed by
the Maxwell stress tensor : en.wikipedia.org/wiki/Maxwell_stress_tensor
@@SimulatingPhysics But if light has no mass how can photons have mass or momentum? Am i looking at it too simplistically?
@@scottdc2105 A particle without mass can have energy and momentum. The classical formulas (p = m*v , E = 1/2 * m *v^2) are not valid for particles with speed near the speed or light or with the speed of light. Instead we need to use the quantum and relativistic formulas: p = h/λ , E^2 = (p*c)^2 + (m*c^2)^2
@@SimulatingPhysics nice
I'm so glad to see fermilab explain this so acutely! It's all about the capacitance of the glass to store the electric field of the light, counteracting it, and finally returning the stored energy of the light upon it's exit.
Sir , you are a LEGEND.
This is the best explanation of refraction ever , in my graduation i learned this E perpendicular and E parallel but understood it today after 9 years, thanks for making such a video. Love from India🇮🇳🇮🇳🇮🇳
Great video. This channel is a real public service
Thanks for clearing up a misconception that was taught to me that I've been passing down to my students. Why doesn't the parallel component have to decrease, I missed that.
Same here, wondering why the parallel one did not decrease
Because both initial components are actually derived from boundary conditions for M and E part. So this is only valid at entrance point. Further on, the wave propagates straight through that medium and both components are affected by the medium and as long as the medium remains the same, it behaves like perpendicular entrance to that same medium. Otherwise the light would curve in that medium which is not the case.
Apparently the ‘epsolom’ (sorry don’t know spelling) is bigger in glass than in air.
This effects the perpendicular component. (I’m not sure myself why this doesn’t affect the parallel component too. Apparently it’s just the perpendicular component 🧐)
Because of this, the angle of the perpendicular component is different to that of the parallel component. It is made shorter
This creates the angle we see when light his glass.
You didn't miss it. He didn't explain it - he asserted it "from the calculus".
He didn't explain it. 12:22 He showed E field entering perpendicular to the surface, thus the electrons in the glass lined up horizontally to cancel it, so he 'cleverly' deduced that the perpendicular E component got reduced in the glass.
If E field entered at an inclination (which should be when light entered at any random angle), then by same logic, electrons in the glass should line up in appropriate direction to cancel it, so both parallel and perpendicular E field components should change.
Light bends because it is weak on the knees.
Fookin Kneelers!
You are the weakest link, kneeeeeeeee'!
Forgive me, I don't understand why the horizontal electric field doesn't lowered in the same way as the vertical one
I think it's because the beam's vertical electric field is moving against the intrinsic electric field of the material, whereas the beam's horizontal electric field isn't.
I'm only guessing though, and I'll defer to anyone who actually knows what they're talking about.
Look to the equations, he showed it
He did not show it. But said that we can get there after some calculus. The so called boundary conditions between two media are shown in em.geosci.xyz/content/maxwell1_fundamentals/interface_conditions/derivation.html
It uses the Maxwell equations mentioned in the video, but in its integral form.
@@saspinski That's the math. It doesn't explain the physics. You would think the charges in the material would get polarized by the parallel component. The dielectric constant is also just an approximation, because it assumes the material is continuous.
To fully appreciate this result, you'd need to read the electromagnetic theory. The electromagnetic theory says that light is an electromagnetic wave, it is nothing but oscillating electric and magnetic fields. Electric fields and magnetic fields satisfy a set of 4 equations, 2 for electric fields and 2 for magnetic fields. The 4 equations are different for different media or materials. But at the boundary of two media, both equations of medium 1 and medium 2 must be satisfied, because the boundary belongs to both the media. So the electric field equations in one medium must be equal to the electric field equations in the second medium. This is what we call a boundary "constraint". The electric field is constrained to move in such a way that the boundary conditions or boundary constraints are satisfied. It turns out the boundary constraints you get on solving the equations is that the parallel component of the electric field must not change, and the perpendicular component must decrease. Please not that it is ok if you fully do not understand this, this is something that is taught in undergraduate physics course.
I'm an RF engineer and this is one of the best electromagnetic videos that I've seen.
Thank you for this explanation, I was taught the soldier reasoning ( 1966) at school and it never seemed plausible to me. I have no formal science education except high school science but I am an avid follower of all science ( subscribed to the New Scientist magazine for nearly 50 years ) and now subscribe to quite a few science based channels on the internet. This is a great time to be alive !!!!
Everytime when he says "Physics is Everything", that brings a smile on my face (being a student of physics maybe !)
Yes, when you study differential equations you imagine the whole world is an equation. Preposterous of course.
@@nosuchthing8 Whoooaaa
Yeah 😀
Now THIS is a PROPER explanation. None of that 'Smoke deGrasse' brushover. Thank you! Finally I found the answer. Had to go to the top - Fermilab.
Nice seeing someone else that doesn't like how Neil sells a lot of physics concepts
This video is not very long, it is long enough.
like our good buddy Einstein said: Everything should be made as simple as possible, but not simpler.
formolzinho a teacher once told me when I asked how long my paper should be his answer was that it should be like a skirt. Long enough to cover the subject yet short enough to still be interesting.
Barely long enough! I'd have liked it to be even more in-depth. Then again, not sure if I'd have clicked on a 40-minute video.
@@TuttleScott Omg, how come I have never heard that one. I love it
anyway I don't understand nothing because in genere physics today are made to don't understand nothing because instead to be something fun and relaxing is contrarity like a very long run of a deer in a forest.
everything can be very clear if you want tolearn something but to learn something what is not so precious like a new form of energy bec ause this planet must clean herself from many forms of pollution.
I heard light people don't knows what is it.
I have derived those two results about perpendicular and parallel components in my electromagnetic field theory class but never thought it had so much connection with light bending...i wish my professors were like you to tell the real stuff and not making me derive these results for mere marks in exam.
Yeah, that has always been a problem for me. I'm fine with learning equations and seeing that it works, but I want to know how it works, otherwise I'm not understanding a god damn thing. 'The sun shines' is not an explanation as to why it emits light. Might as well say god diddit and here's an equation that can calculate the angle when it hits glass.
Talking about radio waves: These can be bent and change polarization in electric fields - think antenna theori. So that should also be possible with light? Is there any examples of bending light or changing polarity shining it between charged plates?
This is the BEST explanation I've seen thus far!!! And I used to teach Physics for 20 odd years!!!
And the fact that one can derive this result purely mathematically, using Maxwell's Equations, makes this a clear winner!
This upload: *Why is light slower in glass? - Sixty Symbols* did a pretty good explanation using the idea of electromagnetic waves, but then it got bogged down with the chaotic and cumulative effects of the quantum electromagnetic nature of the wave's photons, interacting with the quantum electromagnetic effects of the atoms of the medium, as they too oscillate and produce such waves themselves as photons pass near them.
Indeed, a new 'particle' to describe this phenomena: *the Polariton* was used in an effort to explain what might be happening in a quantum mechanical sense.
I'm sure such effort have their merits, even though the maths involved must be INSANE so, this very simple approach in relative terms, using just the classical laws of Maxwell's Equations, for me makes this explanation the clear winner!!!
I've watched a LOT of people explain this concept wrong, finally someone worth their weight in gold
Did you know that first explanation was the inspiration for the short story "Story of your life" which was made into the movie "Arrival?" It was somehow explained tot he author that light must somehow KNOW what the shortest path would be when going through a medium change. That meant that light had already seen the future. I guess it was really Variational principle in general, but the light thing caught him first.
There are differing motives when explaining physics:
1. Describe what happens in a way people can remember WHAT happens.
2. Describe what happens in a way people can have a rudimentary understanding of WHY it happens.
3. Describe what happens in a way that only physics students can understand.
I like that you took a middle ground between 2 and 3 in this video. Explaining physics to non-physics students is tricky business. :)
But can you prove existence and uniqueness of these motives?
12:17 Isn't the arrangement of charges wrong? I mean if the glass particles were to generate an electric field in a direction opposite to the total electric field then the -ve ones must be above the +ve ones because only then we would get a field opposite to the total electric field.
I agree
You have made a complicated phenomenon look simple, but I can understand the great effort behind it. Thank you Professor
I searched a lot for this question but no answer was satisfying me, but your explanation is amazing, I never ever even imagine that the knowledge of electromagnetism is behind the bending of light
This answers one of my party killer questions i asked for almost 20 years. Thank you!
Is there a video explaining reflection?
after watch 2 videos, i think electric part of light is not enough to properly explain reflection
think about it. reflection need parallel and perpendicular part of electric field to flipped 180 degree AND material generated electric field stronger after hit by light to make electric field flipped
someone can correct me if im wrong or maybe have better explanation :D
I think reflection is explained by the fact that light is changing magnetic and electric field not just one or the other, the change in electric field gives rise to magentic and visa versa , in the reflection case it the electric field goes to zero as it is completely completely reflective material and hence the change in electric is negative which produces magetic field in the opposite direction which then induces electric field in opposite direction hence reverse the perpendicular component of the field while not affecting the parallel one and effectively reversing the direction of propagation in the perpendicular direction.
@@vik24oct1991 Okay, wow. I have to read this a couple of times. But thanks!
So have experiments been done with varying electric field intensity in glass/water to see if that changes the angle?
They never taught me this..glad to be here🤗
About Maxwell's equations: "I let you the derivations because that's the fun part".
Thanks !
What a spectacular video, the explanation, the graphics, the editing just perfect. If they taught science like this is schools, everyone would go for science
I am a new teacher covering the topic of refraction for the first time.
ALL of my resources talk about the "Marching soldiers" theory. I thought I was going crazy.
I ketp asking "But WHY?!"
I really appreciate this explanation.
And yeah, almost forgot. This wasn't a long video. I really like good explanations. A good explanation deserves ALL the time needed for that. I wouldn't mind longer videos in the future if it is required.
I'm subscribing cause I love physics and this channel makes me love physics even more.
In the animation, the electric charges of the molecules in the glass moves/reorient themselves according to the direction of the light, but light travels through my window from many directions at the same time. So please explain how each light beam can bend when it transitions from air to glass, consistent with the equtions. One would think the charges of the molecules in the glass gets confused by the bombardment of light travelling through it from various directions?
BTW, I love your videos - sooo much better that what you stumble upon on the web.
What a hard question! I'm delighted that you have asked it. May I say it's all about the supremacy of geometry over physical mater. Any model that includes light waves or rays, and in particular the directions of these waves and rays is wrong by definition. Every such a model is by necessity a 3D structure displayed in an instantaneous space. It doesn't meter if these models were kinetically animated, they are still only the sequences of frozen 3D frames. BTW, the eternity could be the three dimensional only. To be able to answer your question one have to realize that there is no underlying reality of pictorial, i.e. geometrical, models and they should be looked at as if they were eternal objects themselves. However, there is no such thing as the 3D eternal model of light, or of electromagnetic extension in general. Light does have a peculiar direction which is always toward the observing, receipting, point. If you are interested in ...
Respect
The equations are linear so the solutions just add up. Look up 'superposition of waves'
I've learned this derivation at university and I remember that a key part in the derivation was that it is impossible for the electric and magnetic fields to exhibit discontinuities except for the case of a superconductor. Could you explain why the fields need to be continuous and why they are allowed discontinuities in superconductors?
Waves are continuous and superconductor
Surface capacitance tensions
No gravity Lol just guessing
For fun
@@stevesastrohowardkings2245 ?
@@stevesastrohowardkings2245 ??
@@stevesastrohowardkings2245 xDD
What I needed here was for the sound and sight of your explanation to slow way down when it entered my head so I could understand it.
I wasted a lot of time trying to understand explanations in undergraduate physics books. Often it just was too contrived. Thanks to channels like this, one gets real insights.
"Don't forget Uli's goodbye cake! 2:30pm"
I don't know why that cracks me up. Dr. Don, you are freaking awesome!
The cake is a lie
@@Trias805 you're a lie
At 12:05, shouldn't the charges move around in the opposite way? Positive must be downwards and negative upwards.
BTW thank you so much, watching your videos really help me understand Physics better.
Q: Why does light bend when it enters glass?
A: Because it is a pane in the glass.
What was the last thing to thru the bugs mind when it the glass windshield?
Its rear end.
Why?
Because he wasnt light enough.
This is exactly how I wanted my questions to be answered. I appreciate it!
Never got this explanation ever before. Thank you sir.
Wow! I love how this channel approaches seemingly complex physics, with easy-to-understand explanations. The real mechanism for the bending of light is really interesting. But it creates several other questions:
1) As molecular rearrangements differ between different facets of a crystal, does this mean that different facets of crystals have different refractive indices due to different electric field environments?
2) Does this mechanism also explain total internal reflection? Or to be more precise, how does it explain total internal reflection?
Very nice video. The example shown is for TM polarized light. Given that the boundary conditions are quite different for E and B, it'd be interesting to explain why we obtain the exactly same deviation for TE polarized light :)
You dont need any specific form of boundary condition. It is entirely generic. Check Griffiths.
Best simple explanation based on true theory. You're great!
I am really astonished as a physics teacher.Exellent explanation.i am highly satisfied.For the past 20 years I simply explained by mathematical expressions.Now prof explained with electric field concepts is seriously amazing
As a gemologists this easiest and best explanation I seen on refraction
Thank you for the lovely explanation. I have a couple of questions for you and would be grateful for your thoughts:
1. Why doesn't the parallel component polarize the charge/substrate molecules? I mean what's happening physically that prevents that, not mathematically?
2. If I pump in external charge to a substrate using something like a Van de Graaff generator, would it change the permittivity and hence the ref. index of the substrate?
Woah fuking great comment
I subscribe to that first question. All i can find is math/physics hellish formula stuff.
There's a readable explanation of the quantum kind in Feyman's QED book. This video gave the best classical explanation that I've seen.
What page?
@@bradbadley1 You can also watch his lectures, google "feynman new zealand lectures"
Thank you for telling us this.
we're waiting for quantum explanation :D
yes please!
in quantic terms is because of Electromagnetically induced transparency
You have to wait... someone is in the back room inventing long math equations. The show will start in fifteen minutes and you will be dazed and confused. A great show !!!
I am waiting the explanation for TE polarized waves before!
A video from Fermilab offering a classical explanation instead of a quantum one. Oh the irony.
After seeing your explanation many other videos also appeared in my feed telling the reason why light bends?
Now I understand why it was more important to know the wrong explanations!😀
Thanks!!💐
You don't know what I feel.
It's really awesome...
Thank you so much for this explanation.
Love from india
So many insightful comments. Glad to see so many people thinking
So Electric field due to matter in glass makes electric field of light less. Does that mean we can actually bend light with strong enough electric or magnetic field?
I don't think so. physics.stackexchange.com/questions/67464/can-a-light-be-bent-by-a-magnetic-field
Reading the right answer:
physics.stackexchange.com/a/278419
Yes it can
Despite what some equations will try to tell you the answer is NO - there are zero examples of being able to bend light with either electric or magnetic fields. The ONLY example involves a medium such as oil, in which case it is the polarisation of the medium that changes, not the light itself. Don't believe equations, they can explain anything, instead look for real world examples - there are none.
absolutely yes
@@wd41 I hope Fermilab makes video on this one.
12:47 I assume the re-arrangement of the electrical charge takes some time.
So what happens with the "first" photon hitting the glass. Is it going in a straight line because of the not re-arrangent electrical charge?
Or is the re-arrangement itself already influencing that photon since the re-arrangment itself needs energy to happen? How can that influence happen over a distance if the light is going by c?
Greetings
Do you have the explanation now..I really want to know
@@TheAbizgreat10 as far as i know that actual light, or that specific photon will be absorbed by the atom, but now the atom is excited meaning it has more energy wont be stable at that higher energy level, so it will re-emit the photon, but that is not physically the same photon but just the copied version of the original, so it looks like light comes trough glass, but its just the effect of it what comes trough.
photon itself in theory moves in a circular shape, which has its own wave like motion, but when light comes trough glass its a different kinda wave, its more classical physics kinda wave effect, that one atom reemits a new photon, than that photon will travel with 300.000 km/s but only until its captured by the nearest atom, then again atom is excited but not stable, reemits photon again, so the light travels 300.000km/s all the time when its actually travelling, but absorbing and re emitting takes time, and that will make it look like that light is slower now, but its just a kinda trick
this wave effect is now similar to the sound wave, where the sound is not actually gong trough stuff, its just the effect of it appears the other end of lets say the glass door, so u can hear a person talking to u and u can see them trough a glass door, but how the photon is actually moving in vacuum is a different story , probably no one knows how s that happening on its own, cause no one knows what the electromagnetic field is physically
@@fifferfiffer2 Hi. I think you need to see fermilab's original video on "why does light slow down in water?". You'll probably find it interesting
Here he has treated light in a "classical" manner, like a wave. The whole "photon" (particle) interpretation and analysis does not apply in this case. To understand it in terms of the direction a photon is moving we would have to use quantum mechanics equations and principles like the uncertainty of position-momentum.
I don't understand Physics but I am intrigued by it. You just made it understandable. Thank you!
YT should reveal the average duration and average number of times a video is viewed, backed up and o/w actively engaged. Thank you, FermiLab! Everything IS Physics!!
Ok, this is a good classical explanation, but let's go back to Fermat's principle for a moment.
In particular I'm thinking of the explanation given in Feynman's little book "QED" (which I recommend to everyone's attention).
Now it's true that Fermat's principle (i.e. of "least time"), as stated in the video, doesn't actually provide an "explanation". But when combined with the path-integral of the wave-function of the light Fermat's principle simply falls out of the physics.
(Now, I want to clear that my respect for Dr Lincoln is absolute, and I'm perfectly willing to accept that I'm missing something here. But i'm not sure what that is.)
Least action of path integrals is a real and important thing, but it's pretty non-intuitive for a 12 minute video, especially where a lot is debunking common explanations.
Still, the Fermat's principle on its own doesn't explain anything. Feynman's path integrals are really not a common knowledge that would be used in "dumbing down" the underlying causes for light bending on an interface. Being a _part_ of one of the possible solutions doesn't make the Fermat's principle a correct explanation.
@@drdon5205 : Perhaps this should be a request for a discussion of the path-integral view of QM? ;-)
@@Tomas.Malina :
> Being a part of one of the possible solutions doesn't make
> the Fermat's principle a correct explanation.
Um.. of course it doesn't Which is precisely why I made this point in my comment.
I was wondering about TE waves in the video explanation... For TE waves there is no perpendicular electric field yet the wave still bend... Did I miss something or the explanation is incomplete and fails for different wave polarization?
Thanks! Great video!
Reminds me of what one of my professor told me long ago. Classical geometric optics is just Maxwell's equations + matching boundary conditions
Best explanation ever... Physical intuition+mathematical derivation. Thanks a lot!
Thank you! I love how you explain why the misconceptions are wrong.
This might be my favorite physics channel! Awesome work!
Huygens principle gives a unique solution when you integrate, and not just draw diagramms...
Huygens principle is the best explanation of water wave refraction.
@@isaacli7657 And sound waves as well.
Lincoln's Huygens explanation misrepresents the theory. It's terrible, and it's because his wavelets originate only for each new wavefront impinging upon the interface. This is not Huygens idea though. Huygens ' wavelets originate at every point along the interface for each wavefront. The entire point is that all of Lincoln's extra wavefronts don't exist at all according to Huygens, because those are decoherent.
Thanks for this explanation. Could you do a video about why light splits into its component colors when passed through a prism?
It is because frequency of oscillating electric field is different for different colors, so you get different values of perpendicular and parallel oscillations, causing that slight change in direction for different colors (hence separating them)
I would just add that the plane of the exit surface of the prism is a factor. Leaving the glass through a surface angled from the entrance surface, the mix of photons that entered, exits on different vectors organized by frequency. With an exit plane parallel to entrance, like a windowpane, they take different angles through, but individually reverse by the same amount on exit, so they all end up back on the same path (as undifferentiated white light).
extend the same theory to different components of light as they have different wavelengths and hence different refraction angles
Because refractive index or the epsilon depends on the wavelength. For different color of light we have different epsilon so the extent of their bending will also be different. That's why they separate from each other!
Thank you very much. That makes a lot of sense. The way physics taught at school is so dramatic. They could have taught relativity and Maxwell's equation to us first and build other knowledge on it, but they rather create the suspense and mess up our understanding in high school, and only give us the big reveal in the very late stage of university or even higher education. I appreciate the setting, but it took way too long, that just renders any good story incomprehensible.
I have been searching for this answer for the last 5 years ...
Now I have finally got the perfect explanation 😍🥺
Great job, I love this channel, knowledge on the highest level that I can fully trust.
I've been looking for an answer for this question after watching last video but nothing convinced me. Now I know why :)
P.S. I also like your sense of humor, and I'm the person that prefers logical explanations than equations :P
Perfect explanation!!! Thanks for this video. Maxwell equations give exactly the needed framework to explain most of the phenomena. These completed with the boundary equations and field conservation principles are a really elegant way to solve complex problems.
Then tell why sound wave also bends when travelling from different denser mediums
Huygens principle applies to refraction of wave nature of light and also any waves exhibiting refraction including sound
Does gravitational lensing effect work on sound wave?
This channel is well above my pay grade but I enjoy trying to follow along anyway.
Ima go out on a limb and say this video is timeless. It will inform, teach, and enter people's minds year after year upon one man's discovery of the hidden gem in the the galaxy of social media. Forever is forever.
I was about to ask what an Epsilon is and then you explained it... Nice... Thank you...
It's a Greek letter 😎
ε is usually called permittivity which is the ability of a substance to store electrical energy in an electric field
@@KelfranGt yeah, I know that, was just joking/trolling 😇
@Gabriel Rosa I wasn’t explaining it to you, I just wanted to say its other name because saying “epsilon” kinda annoyed me lmao..
@@KelfranGt oops, sorry, my bad 😅
unless the glass has isotropic properties, also parallel electric field should induce charge orientation on the horizontal axis, thus the parallel field should be dumped by the inducted field... but this violates continuity of the electric field on the interface... so where am I wrong?
12:11 sir how does the charges align this way when the external E field is directed downwards?
this is a good question, which the video completely hand-waved.
I really like these videos. The two that deal with light's speed in water and how it bends were very enlightening to me. I'd also like to see one on how light reflects from a surface at the same level as the other two.
damn. This was so much needed. I had been searching for this answer. I almost had given up on this. Before this, I had an impression that it follows the marching soldier analogy with the fixed line, the only difference was I assumed that fixes line to be electric field but finally got the correct explanation.
Thank you for the detailed explanation on the physical nature of what Epsilon means. I'm curious to know how the Tand Delta (or loss tangent) relates to this? Specifically I would like to know how does the energy in the field gets converted into heat within die glass. Thank you in advance.
Dr. Lincoln, would you please make a video with the quantum explanation?
don't Ruiz everything.
I recommend reading QED: The Strange Theory of Light and Matter by Richard Feynman.
Thank you.
But his answer is similar to Fermat's principle. But he basically says path integral of everything but the fastest path cancel.
@@joshuascholar3220 Yeah. The multiple solutions mentioned in this video actually *are* physically real, if they don't cancel out with other such solutions. If there's physical structure with details on the same order of magnitude of the wavelength of the light, you get diffraction grating, thanks to all those multiple solutions cancelling out differently. That's why optical disks are rainbow-y. Oily puddles get the same effect from how thin the oil layer is on top of the water.
Man that's a good book. I need to read it again.
Holy cow, I just picked this up and I'm super stoked to give it a read.
When I found out that a beam of light travels towards the normal when moving into a material of higher density, I came up with the marching soldiers explanation on my own. I don't understand the physics to fully grasp the real reason light bends, but I'm so thankful for this explanation!
May I ask why the counter electric field in the material only happens in the perpendicular direction and not in the parallel one? Thanks
charges shown in the glass are opposite as it has to be,(+should be replaced by - and - by +)
Yes
Dear Fermilab. This second video does not really tell the story either. This is just another way of deriving Snell’s Law. The crucial point here is why only the normal component of the E field is affected by the permittivity. The “real” reason for the behaviour of light in a medium in this context is that light represents a force that in turn induces dipoles in the medium. This gives rise to a London force that always is attractive. Any component of the attraction other than the normal cancels out by an opposite force on the other side. Think of concentric circles on the surface around the focus that each result in an attraction. The larger the radius, each circle have more slanted induced dipoles. This is also the reason that the reflection results in polarization effects (Brewster). It is illustrative to imagine an object on a transverse wave. In an idealized setting the object will move only up and down - there is no sideways motion. Likewise, the London force resulting from the incident light will only affect the normal component (of the EM field in this case).
@@no-one-in-particular Finally somebody says it. I felt the same way.
Hi, I found your comment the other day and it seems like the answer I'm looking for but I've been having a hard time getting at what exactly is happening.
I think what you're saying is, look at an atom at the top "layer" of glass that's been polarized by the light and turned into a dipole. This dipole will polarize its neighbors. The neighbors on the same layer should apply a force to stretch that dipole out more in the transverse direction, but their forces cancel. The neighbors on the second layer stretch the dipole in the normal direction, and this force cannot be cancelled since there is no layer above the first.
This almost seems to work out, but if the transverse forces cancel out and the dipole remains polarizes in the transverse direction, that component of the electric field would also decrease
Exactly what I was looking for, Thank you! And the video is high quality, looks and sounds awesome
Ok, can't reattach my jaw now from how mind blown I was by this explanation
That was a great explanation. Now I would like to know, why some materials let light pass, some absorb light or some reflect light. What is the physics behind THAT?
Matter is made up of atoms, atoms have electrons in specific orbit(al)s around the nuclei, each orbit has a specific and fixed energy gap with other orbits. Suppose there are two orbits with energy gap of E, and you supply exactly E amount of energy to an electron in the lower orbit, it will take it and will jump into the higher orbit, if you don't give that specific energy E, little less or little more and the electron will not accept it! Now, lights come in different wavelengths and hence energies, some lights will have that exact amount E and that specific atom (of that matter) will accept it (absorb) and will get excited... you send light of slightly different energy and it will just pass through (refract). By the way, all that refract, they reflect too, in general, they scatter light....
@@atanunath Hi atanu, I am confused with the absorption thing. Is absorption always related to transition? I ran into some thing about absorption to x rays as a function of the x ray energy. It says the absorption decreases with the x ray energy gets higher until it reaches a specific energy under which the inner electron transition happens and abrupt increase in absorption occurs.
I knew that absorption happens alongside the transition of electrons, but is all absorption linked to electron transition? What happens during the decreasing region I mentioned above.
English is not my native language. Hopefully I pass my idea clearly.
@@linseito2530 if you can provide a link I can try to explain
Thanks, very nice. question: What if the light is polarized // to the interface (E does not have a perpendicular component)? Then B will have to adjust I suppose?
If I understand your question correctly, if the light is polarized such that the E is field is parallel to the interface then then beam is entering the glass perpendicular to the glass (an angle of 0 with the normal), and the beam will move into the glass and NOT change direction. The light will slow down (see the video he refers to at the beginning of this video).
Mr. Swaney, not in general. In EM waves, E is perpendicular to B and also to propagation direction. there are two basic possibilities for E. The video uses so-called s-polarized EM waves. In s-polarized waves, B is // to interface and E is perpendicular to B. In p-polarized waves, E is // to interface and B is not. You may just search for "polarization of light" for explanation. for p-polarized light, I guess one may invocke the continuity of Maxwell Eq. for B at the interface and get the same explanation for the bending.
@@Hiking-guide-and-scenery, ah, I think I understand your question now. What if the E field is directed into/out of the screen as we watch the video. Good question! As he said at ~9:08, he was only going to focus on the electric field. But it seems even then your question still applies.
@ Mr. Swaney, Yes that's it E into or out of page. well, I suppose he would change teh explanation from 9:33 and say "we keep only the 2 top equations..." But it is easier explain with E field as he says, because the B field would involve current loops~~