The Mystery of Light - Walter Lewin - July 19, 2005

This was a talk for a very small group of high school students and science teachers. Prof. Lewin talked about the bizarre behavior of light and he did many demonstrations to show how bizarre light can be. He showed interference of light (also of sound). Everyone was given a diffraction grating (to keep) with which they could see spectral lines in helium and neon. He also demonstrated polarization of light.

[@flxblflyr]

When I was about 10 years old, maybe 1970, my Dad saw a notice in the paper, of a visiting scientist at Furman University, near our home. We went see his public lecture on X-Ray astronomy, not knowing what to expect. The charming, enthusiastic young lecturer told a wild, absorbing tale of his adventures with balloon-borne telescopes in the Autralian outback- and what they had learned about X-Ray objects- neutron stars, black holes... It was thrilling; I was rapt. I never forgot the feeling of being at that lecture. I went on to become an electrical engineer, and never lost my love of the science. Forty five years later, I found Lewin on you tube, as I was preparing for teaching some high schoolers. I started watching his lectures- which are exemplary. After quite a few, I came to his last lecture- where he reminisced about his early research at MIT. X-Ray telescopes, ballons, Australia. Oddly familiar...When he showed the photos, I suddenly realized I'd seen those pictures before- when I was ten. The guest lecturer who inspired so much was the young Walter Lewin. Thanks, Dr Lewin.

[@lecturesbywalterlewin.they9259]

hello Stan. I recall that I gave 5 lectures at Furman every January for 5 years in a row. The lecture that you attended may have been just one lecture before my January routine returns.

[@jjtompson5914]

@@lecturesbywalterlewin.they9259 I have question if i may? How can both statements below be true? Knowledge of the Which Path collapses the Wave Function.....................(DCQE Experiment) Knowledge of the Which Path does Not collapse the Wave Function......(Observation of single slit diffraction)

[@81giorikas]

@@lecturesbywalterlewin.they9259 Didn't Einsteing himself actually hinted on the answer to the double slit experiment anyway? Not so much of a mystery the way he thinks of it.

[@maxmax0]

Wow! Quite a story!

[@Todenkopf09]

I wish my Physics professors had this kind of passion. I love seeing the methods for discovery on these topics, the reasoning behind why they attempted different experiments. One answer can lead to so many more interesting questions.

[@lecturesbywalterlewin.they9259]

Kurushimu :)

[@sosheeanand3537]

Dear prof Lewin I am a researcher with PhD in Biotechnology. I ran out from Physics and Maths because it was so difficult to understand things. Ironically I landed in a Physics lab in Grenoble, France for my post doc (protein biophysics). I saw your "last lecture" first and my mind blowed as things were explained so simple. I started watching your lectures every weekend and now I am confident that I understand things. Extending this now I am also learning astrophysics and calculus. World (at least for those countries with poor education) need inspiring teachers, I blame my country for not giving this kind of basic education. It is more important not to destroy the interest by bad teaching but this is what going on everywhere. Now I can inspire my 7 year old son to study Physics and Maths..

[@lecturesbywalterlewin.they9259]

+Soshee Ananda Thank you Soshee. I am delighted that my lectures have changed your life. I hope your son will also learn to appreciate them in time.

[@aruntejamarakani6817]

Lectures by Walter Lewin. They will make you ♥ Physics.

[@animations3193]

Man thats ur fault ,dont blame india

[@mitochondria7321]

@@animations3193 yes ,not every indian physics or maths lecturers have ability to lecture like this

[@animations3193]

@@mitochondria7321 man don't think like that, there r so many proff's who can do teachings better those mit proff . Im not blaming mit but some concepts mit proff not explained better than my proff.

[@Meninx87]

Professor Lewin you are truly an inspiration and i'm grateful to you for the work you've done.

[@lecturesbywalterlewin.they9259]

+Meninx87 Thank you Meninx

[@muhammadirfankhan997]

Sir Walter Lewin, I have made a habit of watching at least one of your lecture a day and getting amazed by the beauty of Physics, thank you so much. Though I remained a regular student of Physics from schooling to university but understanding it to its core was never this simple. Wish I was able to take one of your lecture if not all. Also, I am buying your book "for the love of Physics" as a reminder of your great work in the field and in the meanwhile requesting the administrator of this page to upload every lecture of yours here on youtube. Thanks and stay healthy. :)

[@lecturesbywalterlewin.they9259]

+Muhammad Irfan Khan Dear Muhammad. Thanks for your kind note. There are 284 videos on this site. That will keep you busy! Good Luck!

[@ahsanrubel2869]

I was not a student of physics... We were so unlucky that we never had teachers like Walter Lewin. It’s surprising that I found myself learning physics because of this kind of teachers.. Long live sir Walter Lewin..

[@cmetube]

Great lecture Professor Lewin. Beautifully organized and incredibly poetic

[@susansutton1712]

thank you for making complex knowledge easy to understand and for making it easy to access your lectures. Much respect to you Sir Walter Lewin 🖖

[@christopherwoodcock8535]

Second time I’ve watched this great lecture. Since the first time I have purchased a Linear Polarisation Filter of my own. Fascinating stuff. Thank you Professor Lewin. Inspirational as ever

[@amisharawal3967]

Hats off Professor Walter Lewin !!! I m in 11th standard, taken PCM (science - A group), very interested in and fascinated by physics (especially quantum).... I clearly understood 95% of all your lectures........... Very inspiring, and in a nutshell, I can say, your lecture is like 'Physics Made Simple'............... Thank you sooooooooooooo much for sooo very much interesting and inspiring lectures............

[@tylershepard4269]

Hi Professor Lewin I’m an undergraduate in Electrical Engineering studying RF and Microwaves. Thank you for helping me to love light!

[@satyanaidu4]

A bad teacher complains, an average teacher teaches, a good teacher explains, a great teacher inspires! I'm writing this at 4am, you sir have made me decide to pick up my physics textbook again at age 36! Thank you!! :) p.s: I love it the way you say " this is an illegal question!"

[@quantum7401]

+ Illegal question

[@kenantahir]

you are an awesome teacher. i only wish i had teachers like you back in school days (20yrs ago).. maybe then i would have passed my physics exams and would have had interest in them back then... see this teacher knows how to break it all down to the last bit.. he practically unravels it all for you to understand!

[@lalropuiachinzah3335]

Since i watch this lecture video, now everyday i watch at least one lecture during lockdown, I highly appreciate and love your teaching,Sir

[@directedbypuma]

You make it so understandable, thank you! I am a licensed ham radio enthusiast and this lecture puts it all in perspective for me. I think a particle is also a wave but at very high frequency that you might not see its wave effects in nature.

[@sambastable513]

I concur, I think the frequency becomes so high that it becomes Circular producing matter

[@chekystar]

great teacher i am not student of physics but i watch your lectures anyway because you make it interesting and because i love to know how the world works. thank you :) i learned a lot from you. keep on the good work

[@jessemontano6399]

Wow, it's so awesome that the professor answers so many questions/comments. That's legend!!!

[@manishk45]

I would like to meet this #Physicist once in life. I love his lectures.

[@mindscraped]

i posted a question about one of his light demonstrations and asked a question about the light being a wave or particle and the observer effect . You guys gave me an amazing answer i have never heard before thanks. But i am commenting about it here about that post because i cannot find it anymore. I just know i got a response with my notification . The point is thank you so much for answering my post . the answer was amazing

[@swapnilsaha8143]

Dear Professor Lewin I am an Indian Engineering Student... From Childhood I 've always wanted to be a physics teacher..... Its really Hard in my country to Catch This type of dream....Here this system doesnot look How much you know rather it looks How much U have got marks....they will not see... Who has understood and who has only memorized the Whole page books... research...I've had So many questions...I've resolved...many...and still have some...Sir, U probably donot know.... Everytime I see your lectures and remember your picture.... I gain my motivation to get a Stable post in research....Even though I had to choose engineering...because of Our Education system....but Still If I have to go to particle research path...I have to take a very Very long path....And I will obviously succeed...one day....Your Lectures...changed my perceptions and helped me get my motivations back...everytime I've been pushed apart from physics.. Thank You Sir♥♥♥♥♥♥🤣

[@gopisingh1630]

love from india . i live in a village in punjab ,india . here , everyone wants to go to U.S , just to earn more money, by doing labour . but i would like to go there as a theoretical physicist . thank you sir ,for helping me lose my virginity in physics.

[@soumenatha7288]

😂

[@simonmasters3295]

​@@soumenatha7288I found the sentiment rather challenging...do you suppose she or he got to the US?

[@joseg.matamoros2847]

“Lose my virginity in physics” is wild

[@vik2377]

Kudos to you my friend. .keep it up and you would be successful

[@keithlillis7962]

Excellent stuff and so glad Prof Lewin mentioned in a little more detail the findings of Quantum Mechanics, as up to that point he had me convinced that light was just a wave :-)

[@edmundotu4602]

i really love you Walter Lewin, you make everything so easy to understand

[@lecturesbywalterlewin.they9259]

+Edmund Out :) :) :)

[@lbomcarvalho]

Prof Lewin, your performance I take as a standard performance of somebody that loves physics, science, teaching, ..., my sincere congratulations.

[@lecturesbywalterlewin.they9259]

:)

[@suecondon1685]

For me, light is the biggest mystery of all. Thank you, this was fascinating.

[@lecturesbywalterlewin.they9259]

@@PreCognition777 yes, and since the moon is made of green cheese we will even be able to eat the moon

[@seminolerick6845]

Just found this awesome presenter of info, yesterday. On top of everything else... never saw anyone else draw dotted lines on a chalkboard like him ! Silly, I know... but a great, effective tool in his tool box !

[@suecondon1685]

That's another mystery! 🤔 How does he do that!

[@clayz1]

He’s given this lecture hundreds of times. Probably. Very good, I have seen it a couple of times.

[@alimukhtar4759]

Hello professor Your lectures are helping me and many other physics lovers to understand the nature quite easily. I am lucky to be able to get your lectures online Wish you a healthy life

[@bradjunes1610]

It was hard to turn off what I do at age 70 and get back into thinking, but enjoyed your thoughts. I've tout many with a simpler approach of more one on one. The masses won't understand at the speed you teach. But I loved it. Much thanks.

[@lecturesbywalterlewin.they9259]

:)

[@joannadrozdz3848]

Wonderful lecture, thank you.

[@das250250]

Walter not only teaches at the highest possible level and professionalism but his black board layout is so organised , writing is legible and drawings are just so well drawn. In my opinion , he sets the bench mark for lecturers ..

[@lecturesbywalterlewin.they9259]

thank you :)

[@nobodyyouknow9839]

My father give me telescope at age 15 on my birthday 😅..... As I asked him for that later he asked why I am so addicted towards science and physics specially then I answered you don't meet the legend Walter lewin that's why you are talking like I meet him daliy on his KZbin channel and get inspired . Your every lecture is a masterpiece .... I am glad that camera and internet make your lectures alive forever . Wish that I can also witness the great Richard fyemen 😢😳. Love from India sir

[@lecturesbywalterlewin.they9259]

lucky you!!

[@dalenassar9152]

Prof Lewin. Your presentation: "THE MYSTERY OF LIGHT" was fascinating--thank you. I think I have an explanation on why the small audience could NOT hear a VERY low volume with the destructive interference of the sound waves emanating from the speakers (while you did in private). PERHAPS THE SOUND IS REFLECTING OFF THE SWAYING, GROUP OF PEOPLE--THUS SCATTERING THE NODES SOMEWHAT! Thanks for everything--you will never know the enjoyment you have provided me. Dale Nassar, Amite, Louisiana.

[@lecturesbywalterlewin.they9259]

yes sound is sometimes problematic for that reason

[@kss4282]

Please answer me Sir. I am from India. I'm crazy about physics. I have a doubt. If I turn on two lasers (say A and B)facing each other simultaneously in vacuum. Then the velocity of 1st photon thrown from A with respect to the first photon from B shouldn't be half of c. How is it constant?

[@studyingchannel2328]

I also want to know .please like my comment when teacher give you the answer. So I can be notified.

[@MihaiBadicioiu]

Wow - a new "episode" I did not knew.... thank you!

[@lecturesbywalterlewin.they9259]

Mihai Badicioiu This site contains ALL my lectures. My 94 course lectures are in much higher resolution than anywhere else. They also show problems and exams (and solutions) and Lecture Notes (pdf files below the videos). Not all my course lectures have Lecture notes. Notice the many non-course lectures including my 8 lectures that I gave for Japan TV (NHK) these are in HD!

[@rajeshkaushik5223]

I loves Ur all lectures, it's very adorable.u r my best teacher of physics

[@lecturesbywalterlewin.they9259]

Thank you!

[@ajaykumarmaruvada9113]

Dear prof Lewin a very happy teachers day. We wish India should have a guru like you to explore our horizons and to know that science which hides truth is also curious and fun. Thank u once again . HAPPY TEACHERS DAY. !!!!!!

[@lecturesbywalterlewin.they9259]

thank you very much

[@shadowmedow4028]

sir ur lectures are damn inspiring.....whenever i wtch them i find a great diff in my teachers saying and urs...#inspiring they don't give tht chance of questioning topics....they say either ur oversmart or dumbo....and i got depressed due to this reason still i couldn't stop it ....u r a living inspiraations topics r well revised here thnks sir !!

[@anuragmishra2245]

Sir, Why do we use constant of proportionality ?

[@aaronkellner620]

Very well put together lecture. What I have always had trouble with is the claim that light is some sort of EM wave. The implications of that simply don't clearly manifest as one would expect. My opinion is that there are clues that light is more than that. Much more.

[@RaivoltG]

I wish you were one of my Professors! I don't understand much of what you discuss but you are so enjoyable to watch, and I do grasp some of what you say! You are pretty funny too! I liked it when you threw "photons" to your students! The egg on your shirt is funny as hell too! I wish more professors were like you! I'm sure that you are making more of an impact than you are aware of! You will definitely be directly responsible for many good things to come. Your students will be the ones to make discoveries, as I'm sure you have and will continue to. Thank you for the videos, I love watching them!

[@sangitasinghal3249]

I can literally hear the destructive interfere on my smartphone. At .. 57:09 and constructive at 57:13 Thanks Professor Lewin, you really made me to ❤️ Physics.

[@miz2k188]

Ye bro me to o Thanks for this

[@santhosh.thatikayala7479]

Thanku very much sir...u make me love physics yjrough ur lectures

[@sahilkumar-jn8vb]

Excellent lecture with clear explanation sir u r great Watching your lecture I m inspired how to make physics more and more interesting before this physics was just a discription of heavy maths and digest them anyhow to pass to clear the exam Nice sir

[@faisalkhatri1813]

awesome.. Thank you sir.. very well explanation.

[@bulelanigongxeka4468]

No words but wow, I don't even do physics but yeah you're a wiz and I appreciate your zest for physics, you have lived your human life to the fullest.

[@UtraVioletDreams]

Physics I love it. Bedankt Prof. Lewin

[@em_universal]

Awesome explanation!

[@TheZooman22]

These lectures are quite enjoyable.

[@khanyusuf09]

Sir you make physics look so interesting and beautiful exactly the way it is...your way of teaching gives me an insight of how the nature works rather than scratching my head on all that dumb dead physics equations spread all over my physics notebook..you taught me the beauty of those equations...so thank you sir...stay healthy...😊

[@lecturesbywalterlewin.they9259]

that's why my lectures are world famous! :)

[@mavis2701]

i've always loved physics, and Mr. Lewin is a great example of why

[@lecturesbywalterlewin.they9259]

:) 😊

[@drdeak]

Particles or waves, hey professor Lewin what about David Bohm's theories on the argument?

[@amisharawal3967]

And yaa also I have a question......, In some of the many versions of the double slit experiments performed by many physicists, many used electrons, and some of them used single electrons..... But according to the uncertainty principle, we cannot measure both the position, and the momentum of any particle at the same time, accuretely...... So how can we manage to produce (eject) only single electrons of specific momentum, without first locating them in an atom, and in turn ruining the certainty in its momentum ( which is what we actually wanna be very certain about, especially in the quantum eraser version of the double slit experiment) Pls answer and plsssss correctme if I m wrong........ And once again thank you Professor for the lecture........

[@kiprutojunior9826]

The best physics lecturer of all time

[@lemont2005]

Hello! Sorry for this inocent/strange question that I´ll ask: Did anybody have ever try to collide one eletron against one proton? What happened so?

[@lecturesbywalterlewin.they9259]

Good question. In principle this can be done, but you would learn very little from just one such such an interaction. Experiments are therefore always done with large amounts of electrons and protons, not just 1.The results vary greatly depending on the energy of the electron. . "1) Elastic electron-proton scattering: the electron and proton just "bounce" off each other under some angle theta. By observing the cross section of the scattering versus the theta angle it was shown that proton is not a point particle, but an extended object. 2) Deep inelastic scattering: the incoming high energy electron "destroys" the proton into a bunch of outgoing hadrons (mostly pions). By observing the cross section of this interaction it can be shown that proton is composed of pointlike particles. The electrons collide elastically with a parton"

[@kovanovsky2233]

Just to add from what I know, there is a research going on where scientists basically smash 2 protons (if I remember correctly) together and analyze what happens. And that's how they found subatomic particles (quarks, etc). Please correct me if I'm wrong.

[@prakharmathur9453]

I have 3 questions:- 1) Since proton and electron attract each other, then how will they bounce off each other? 2) What are partons? 3) Is the energy of electron in first case lower than that in second case?

[@lecturesbywalterlewin.they9259]

1. learn about QM 2. use google 3. question unclear

[@gh0stgl1tch]

@@lecturesbywalterlewin.they9259 if so then how light passes through glass. Just curious , I think of a lot but everything ends in a stand still

[@dalenassar9152]

Prof Lewin, I have just watched your lecture on the uncertainty principle with the demo of the light spreading as you narrowed the vertical slit. I have been wondering: What would happen if, instead of a shrinking linear slit, there was a shrinking circular opening (such as an iris) that could shrink all the way shut. What would the light pattern look like as the uncertainty spreading began???

[@lecturesbywalterlewin.they9259]

>>>What would the light pattern look like as the uncertainty spreading began???>>> yes, of course UP always holds.

[@dgtamil492]

sir,i like ur all the lectures ,it is very awesome ...............................i love so much ur art of teaching..........

[@lecturesbywalterlewin.they9259]

:)

[@KaleOrton]

Immaculate educator. Thank you sir. 🙏👍

[@fireboymsd7456]

Always delivering explicit lectures

[@glen-draketoolworks7186]

The "orders" appear to fade as they move away from the zero order. Is that really happening and if so, then why?

[@chazguthrieful]

Is the double slit experiment interference due to partical entanglement?

[@hewhenthehe3722]

Professor Lewin, can i just say the wave you drew at 6:02 was beautiful

[@Eztoez]

Professor Lewin, you remind me of Richard Feynman: both superlative communicators and deeply passionate about Newtonian physics & QM. Thank you.

[@lecturesbywalterlewin.they9259]

:)

[@luckynewman]

Dr. Lewin thank you for the videos! Awesome delivery. I have been trying to find an answer to the question how much uv light penetrates water (say pool water) I see a lot of sites saying that 100% of uv is reflected and others contradicting this. If you could shed some light on this question or maybe point me in the direction where i can learn more for myself that would be really cool. Thank you Professor Lewin and my girlfriend and i loved your book btw we got the audio version!

[@lecturesbywalterlewin.they9259]

+Brian Campbell Look at the absorption spectra which are available on the web.

[@princeacez5905]

Can someone please explain how you derive the expression for roughly the number of nodal lines(2d/(wavelength))-39:17

[@dhakshan]

That would be in class 11 or 12 text book right?

[@JimCarver]

Oh good, this one I haven't seen yet. I've watched all the lectures from the three courses, many multiple times. I always catch something I didn't notice the first time around. I just watched your farewell lecture at MIT again. That was kind of sad. I bought your book and am going to give it to my grandson when he gets old enough. That may be pretty soon. He loves science.

[@lecturesbywalterlewin.they9259]

+Jim Carver Hello Jim - If you have seen all my course lectures you should also consider to watch my 8 lectures I gave for TV (NHK) in Japan. The version I have posted on this channel is in English. It's very nice that you will give my book to your grandson. If you send it to me I will sign it and return it to you.

[@JimCarver]

Wow, that's great, Walter. I'll do both! You know, to tell the truth, I had already loved physics...but you made it a lot more fun. :) Thank you and I wish you good health!

[@segalindoa]

The guy sleeping at 27:27 is priceless (also the way the professor draws the dotted lines. That's chalkboard mastery).

[@simranagrawal8441]

Thank u sir for making physics so intresting😊❤

[@jacobmerz2803]

I wish your lectures were captioned for Deaf folks like me.. I watched your last lecture and I was blown away.

[@lecturesbywalterlewin.they9259]

+Lectures by Walter Lewin. They will make you ♥ Physics. Jacob all 8.01x lectures have subtitles. We are also working on 8.02 subtitles (it's time consuming).

[@malvikaawasthi2457]

Does heat+heat also can give no heat if destructive interference is applied please like this as much as so that prof walter lewin can answer this question

[@SriramVad]

Sir, I am really grateful to have come across your videos. They really inspire me. I always liked physics and loved how it manages to develop a counter intuition for a thought. I would like to ask you a question, what are quantum fluctuations exactly?

[@lecturesbywalterlewin.they9259]

use google

[@SriramVad]

this is so far the best reply I've had. XD

[@lecturesbywalterlewin.they9259]

In quantum physics, a quantum fluctuation (or vacuum state fluctuation or vacuum fluctuation) is the temporary change in the amount of energy in a point in space, as explained in Werner Heisenberg's uncertainty principle. This allows the creation of particle-antiparticle pairs of virtual particles.

[@SriramVad]

Lectures by Walter Lewin. They will make you ♥ Physics. thank you very much sir :)

[@MrRichygm]

Another brilliant lecture.

[@lecturesbywalterlewin.they9259]

Thank you Dick

[@MrRichygm]

I can listen to these all day. Only Neil Turock is in the same league as far as I can remember.

[@fjs1111]

Not a typical professor, this is a different scale of talent right here.

[@AnuragMishra-fq4nm]

Sir, Why does mass increase with velocity as an object approaches the speed of light? How can I prove it mathematically?

[@lecturesbywalterlewin.they9259]

E_total=gamma*m*c^2. m does not change, gamma is a function of speed.

[@utuberaj60]

The part of this lecture stating that it is Heisenberg's Uncertainity Principle is behind the fact that electrons, atoms,molecules move continuously and have energy. The same applies to the billiard ball too with an assumed uncertainity limit i.e. h/2*piset as 1, as seen by "Mr. Tomkins in Wonderland"- an entertaining science popularising book by astrophysicist George Gamow. The fact that billiard balls or similar macrocscopic objects don not wiggle around spontaneously again due to the insignificant effect of Heisenberg's Pricnciple on large objects which we encounter in everyday life. I want a bit more of how this Priniciple was derived from De-Broglie Principle and Schrodinger's Wave equation. Hope the professor can show us this too sometime.

[@nagendrasanapalli3200]

Dear professor, I had a doubt that the water wave is travelling by dropping a stone in a river and the sound wave is generated due to the vocal chords and how's the light wave is travelling. And what is the disturbance behind it. Can u please?

[@truebeliever174]

That's the lecture when I was 2 years old. And I am seeing the lecture in 2019. Thanks to KZbin.

[@lecturesbywalterlewin.they9259]

no need to watch it again!

[@drsanjeevgandhi5365]

Can we see objects only because light rays reflected from object enter our eyes?.Also if every object reflects light then why can they form images like mirror do .Hope you answer this .Thanks

[@javariaghafoor1025]

Greetings sir, I wanted to ask whether you have ever visualized newton rings or rings similar to newton rings on the inner surface of your eye glasses (on that side of the lens which faces your eyes) just while walking through the blue on a warm sunny day?

[@lecturesbywalterlewin.they9259]

I don't think I ever did

[@javariaghafoor1025]

But I have sir and it makes me go into deep thought, not monthly or weakly, not even daily, but minute to minute or you may say whenever the sun is shining out there. I have no idea that these are newton rings or some other rings which I may have accidentally discovered. Please guide me and tell that could newton rings ever have a maxima white in colour?

[@lecturesbywalterlewin.they9259]

If there was a thin film (condensation) on your glasses that might explain it. Try to reproduce it. Search the web for an explanation.

[@ramonchan9732]

I like the fact that professor Lewin tried to explain high school physics to those kids.

[@animations3193]

Sir do u know about jest exam for physics

[@rhitabanpal5479]

Hello Sir! I had a question to post but my keypad is limited so I would use ^ for lambda (wavelength) and w for omega (angular frequency) and I would write pi in letters. If I use displacement equation y = asin[(2pi/^)x - wt] then dy/dx = (2pi/^)acos[(2pi/^)x - wt] and dy/dt = - wacos[(2pi/^)x - wt] Therefore, (dy/dt) ÷ (dy/dx) = - v ,where v is the wave speed and this is independent of which wave equation I chose. But again (dy/dt) ÷ (dy/dx) = dx/dt which should be v Is this contradiction purely Mathematical with no Physical significance involved or is there something fishy going on ?

[@lecturesbywalterlewin.they9259]

y=x-2t y is a function of t, x is NOT a function of t. at t=0 y=x straight line thru the origin. At t=2 the straight line has moved a distance 2. y=0 is now at x=+2. The straight line is moving in the +x direction with speed 2. For a given x, y is changing/moving in time. But x is not moving in time. Thus dx/dt is NOT the speed at which the function moves in the x direction. If x-2t = 0, then x is a function of time and dx/dt=-2 Thus the speed at which x moves is then 2 in the minus x direction.

[@yogitadhakad5193]

All ur videos helps alot ....

[@syahrul9282]

I really enjoy watching this even tho im a high school student...

[@SimoneIovane]

I don't know why but when I follow your lessons everything seems so easy. are there any lessons about quantum mechanics? Thanks :)

[@lecturesbywalterlewin.they9259]

+simone iovane To make difficult things easy is my specialty!

[@JohnSmith-jw4wn]

💚❤I absolutely love this❤💚

[@davidwilkie9551]

Life is Quantum Mechanics, but that is a statement about function, not a description about the lifeforms.

[@0Erag0n]

This just shows that Great teachers, make the students. If my teachers was this inspirational i would have loved going to school, and my field of interest would have been much wider.

[@lecturesbywalterlewin.they9259]

+0Erag0n Yes it's cry sad that poor Physics teachers ware the reason that so many students hate Physics. The teachers blame the students, but only the teachers are to blame.

[@JimCarver]

+Lectures by Walter Lewin. They will make you ♥ Physics. Sometimes, or maybe even much of the time, it may be the grad students who put the student off. I remember this one back in college, he didn't want to teach undergrads, he wanted to show us how much smarter he was than us. I've had a few experiences like that in the sciences.

[@ashutosh-ql3gm]

Hello sir i'am from india ,, sir u r a great teacher ,, a real teacher who teaches students from his heart ,, sir in my locality teacher feel shame to teach these basic definition and concepts ,, and if any student ask these type of things they think that this child is dumb n insult him. That child will not able to ask question from next time. Aslo sir u r such a great physicist , even u taught students all concept like student is starting from zero. U r great sir ,, plssss plssss plsss launch a course on physics of 11 & 12 standard b/c sir class 11 & 12 are the pillars of our study career ,, but indian education system not prepare this pillar to stay strong for future ...thank u sir We love u sir 😍😍😍

[@system.machine]

An amusing note. During the "find the dead spot" section, you could hear it as the camera panned across.

[@BladeRunner-td8be]

Learning much from this lecture and have no idea why. lol Differences in wave lengths of different mediums. Destructive interference, constructive interference etc. He has a way of talking down to his audience every once in a while and I have a feeling he's been called into HR quite a few times earlier in his career about this foible he possesses. My prediction is he used to be quite a bit worse early in his teaching career and has gotten much better about it as he's aged. Still his teaching methods are quite good imo and he is worth listening to.

[@rajeshrisadulwad2001]

Sir , I have a query. How can we find the solution to this question? A ray of unpolarised light is incident on a glass plate of refractive index 1.54 at polarising angle , then angle of refraction is? Sir, Please reply.

[@lecturesbywalterlewin.they9259]

>>> at polarising angle>>> what does this mean? Is this the Brewster angle?

[@rajeshrisadulwad2001]

Yes sir

[@vigneshkumar9378]

Sir i am from India, in my college classes seems to be so boring and sleepy, faculties expel what they get from textbooks, it doesnt really take student into his imagination or unveil the mystery of the subject. Your classes are really fantastic sir.

[@sanikabhagwat2253]

Sir, I want to contact you regarding my doubt in K- Electron Capture.

[@lecturesbywalterlewin.they9259]

post your questions here. Make them brief and unique and refer to a lecture and how many minutes into the lecture.

[@josephf151]

As you displayed in your Quantization and Heisenberg's Uncertainty Principle video, as the light is forced through smaller and smaller width slits, the beam begins to stretch due to the uncertainty principle. why do we not see this happening here? Or is it happening, but that stretching is still so to say in wavelength?

[@lecturesbywalterlewin.they9259]

>>>> why do we not see this happening here?>>>> THINKKKKKKKKKKKKK

[@AndersonBeuting]

Professor, is it because in this experiment here you had hundreds of slits (and therefore the momentum of the light wave wasn't disturbed, or with high innacuracy), as opposed to the experiment in the Quantization Lecture, where you had only one slit, limiting the space through which light passed through and therefore altering its momentum?

[@lecturesbywalterlewin.they9259]

I do not know which demo you are referring to. If I used a diffraction grating then thousands of lines etched into the plastic were involved.

[@AndersonBeuting]

Professor Lewin, I hope you are well. I'm refering to this demo right here kzbin.info/www/bejne/g5auYXeMaJdso7c My understanding is that in that demo we only saw the stretching of the beam of light hitting the wall because it was forced through only one very small slit (no diffraction grating), limiting the space the light passes through and changing its momentum. The difference in the video of this page, however, is that a diffraction grating was used, and as you said so yourself, thousands of lines etched into the plastic were involved, so the light's momentum doesn't change, and we don't see the stretching of the light beam. Is this correct?

[@lecturesbywalterlewin.they9259]

I mentioned that I did not know which demo you were referring to. Now I know. I ONLY used a single narrow slit in the demo you had in mind, NOT a diffraction grating that was in a different demo. What you saw was the effect of HUP.

[@blancaroca8786]

At 1.04 we see interference between light waves from a double slit. It is really good to think about what you would get here with only one slit open. Instead of the 9 or so spots of maximum intensity we should get a long swathe of light more or less covering the spots and filling the gaps between with a total light power of about half as only one slit rather than two. We were lucky as we got to do these experiments ourselves in school lab in the 70s. Much better than seeing exagerated cartoons in a textbook. We were lucky our school spent next to zero on textbooks and instead bought some simple apparatus which turns out cheaper and better for learning.

[@chillhopnation7635]

I think why the amplitude doesn't have affect on velocity of wave of sound?

[@arjuns2078]

Happy birthday prof. walter lewin !!

[@chaitanyakasani7718]

Dear Lewin, Can the interference of two light waves traveling in same direction sum up to make standing waves?

[@lecturesbywalterlewin.they9259]

what do you mean by "two light waves"? If you mean light from 2 lasers with the same frequency read this: physics.stackexchange.com/questions/630/is-it-possible-to-observe-interference-from-2-independent-optical-lasers

[@chaitanyakasani7718]

The resonance phenomenon observed in whispering gallery mode resonators (WGM). i'm a bit confused because till now i study that two opposite travelin waves with same frequency, interfernce would lead to standing waves generation. But the standing waves phenomenon observed in WGM is a different.

[@lecturesbywalterlewin.they9259]

Google WGM

[@preranadash9640]

Sir what is the name of the source of helium and neon light and by facing towards the light how one can able to see the patterns ?please sir help me to get the answer

[@lecturesbywalterlewin.they9259]

look at the lights through a grating - you can then tell the difference. for more information use google

[@aadirimal8081]

Amazing!