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

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.

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.

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..

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

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. :)

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............

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

Even old people attend professor walter lectures , this is a proof of how great teacher he is ❤

Great lecture Professor Lewin. Beautifully organized and incredibly poetic

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

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

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!"

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.

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 🖖

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!

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

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.

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

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 !

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♥♥♥♥♥♥🤣

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.

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 ..

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

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

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

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

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 RU-vid 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

Well done Prof lewin 👋👋 You're one of the best physicist I've ever seen in this century

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 :-)

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. !!!!!!

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.

Imagining the multitudes of minds that will make future scientific discoveries this brilliant man has inspired is astounding. Proof that one man's life can change the world. Even if he is not the next Einstein knowing that he may have inspired them and furthered human advancement is amazing and beautiful

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

Immaculate educator. Thank you sir. 🙏👍

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

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?

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

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

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.

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

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

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

Wonderful lecture, thank you.

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

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

I am a physician, Surgeon General, but for very unknown reason I am addicted to these lectures, can any one explain??

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........

De ideale leraar. Geweldig meneer Lewin. Groeten uit Schchchcheveningen

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.

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!

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.

The best physics lecturer of all time

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.

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

I stand corrected, I went back over the videos on Newton and saw why particles and corpuscles cannot be used (as some do) interchangeably. The key was the light experiment.

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

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!

"all I know is what I read in the newspapers" - Will Rodgers - I read through several RU-vid videos and one of them indicated that he was wavering at one time. p.s. Lewin makes great videos

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

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 😍😍😍

Awesome explanation!

Always delivering explicit lectures

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 !!

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.

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

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.

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

i would love to personally be in one of your lectures...

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...😊

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

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.

I lost him at Hello and got it back at Goodbye.

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.

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

Physics I love it. Bedankt Prof. Lewin

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?

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

Geweldige lezing, zeer boeiend en leerzaam. Blij dat ik uw kanaal heb gevonden.

New to your channel professor Lewin ❤️. Take care of your health professor .

i hope i can see and hear personally prof Lewin 😇

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

Thank u sir for making physics so intresting😊❤

Another brilliant lecture.

Is the double slit experiment interference due to partical entanglement?

If sound requires air as a medium, and water waves require water as a medium, then what is the medium of light?

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

Sir, I am currently a graduate student, and I need some serious help. I have always loved physics, and I have watched all your lectures. I actually felt the "love of physics". You have told that "A teacher who makes physics boring, is a criminal". Well, I am trapped between criminals. My professors out here are making physics like eating raw meat. I am unable to tolerate this. Kindly advice me how to survive in such an environment, and to keep the beauty of physics evident for me. How should I teach myself the right way to learn physics ?

Sir you really are an inspiration for many students, I never thought of becoming a teacher but watching you I have the desire to become one. The way you're teaching, giving out your knowledge to the students is remarkable. I'm saddened that not every teacher is passionate like you, often it's not the sibject which is hard or boring but it's mainly the teacher how he's giving the lesson.

These lectures are quite enjoyable.

Prof. Lewin I have this question for a very long time which I'm not satisfied with the answers of my teachers told. "Why does the particle behave as a wave ?" what is driving it to be a wave ? My Teachers : its the property of the particle. Nature loves symmetry. But I'm truely not accepting it, believing there might be a good explanation for that.

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

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

Sir, Why do we use constant of proportionality ?

Happy birthday prof. walter lewin !!

We all are in your debt, sir☺☺

Thanks for the talk Prof Lewin. I got a feeling that light probably spirals from source then within a magnetic field, travelling = three dimensions, and whatever they use to measure the “photon” as it passes the slit, interrupts the field and the particle loses its relative luminescence. You probably thought of that already.

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

Hi Sir. Great lecture. Although I'm beginning to understand why this keeps you up at night. The wave nature of light is a little puzzling to get a grip on. Your lecture demonstrates that light clearly behaves like a wave. What is puzzling is that for water and sound, the wave always behave as waves, ie they propagate in circles and spheres and because of the motion of the wave outwards ie at an angle from its origin, two waves can interfere with each other. However in the case of the laser, the light travels in a cylinder towards the wall, it doesn't behave as a wave should ie it doesn't spread out into a sphere. It would form a circle of light of the same diameter no matter how far the wall was from the laser. So when there is no slit in between the laser and the wall it would appear that light does not behave as a wave. I can think of the cylinder of light from the laser as a sum of an infinite number of cylinders (effectively lines of light beams) travelling parallel to each other. Why then in the double slit experiment do they behave as waves and interfere with each other only when they encounter the slits? Is this because having the wall reflect the light is in essence us determining the position of the photon and hence making it a particle?

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.

Excellent! Simply excellent.