8.03 - Lect 18 - Index of Refraction, Reflection, Fresnel Equations, Brewster Angle

[@LonesomeTraveller]

You are such a jolly human being. So cheerful and optimistic! Bless you, Mr. Lewin!

[@paulg444]

Simply put he is the gold standard for every person that aspires to teach. His impact is great for every age and era.

[@arkadiptaghosh99]

Maxwell's equations may have boundary conditions,but your in depth teaching style knows no boundary,it goes deeper and deeper like a perfect ideal teacher.... perfect conductor,perfect insulator doesn't exists,but in here perfect teacher exists.. example:-WALTER LEWIN,THEY REALLY MADE ME LOVE PHYSICS and most importantly at such a low cost,cost of ZERO...I salute you..

[@lecturesbywalterlewin.they9259]

thanks for your kind words

[@Kavita73]

@@lecturesbywalterlewin.they9259 sir I am a class 10th student and I LOVE YOU ❤️

[@falcodarkzz]

Just wanted to express my gratitude prof, these lectures express the theoretical beauty of physics and it's sheer power through the demos! How amazing, bravo.

[@junwoobak]

It was the best lecture I've ever seen before. Thank you so much for your great efforts. I will watch your lectures again and again when I forgot physics. "Physics make difficult things easy!" I hope you always stay healthy and happy. From Seoul, South Korea.

[@lecturesbywalterlewin.they9259]

Thank you for your kind words

[@marketmail49]

To make impossible things possible... And physics makes difficult things easy! And Prof Walter Lewin makes Physics the easiest to understand. Thank you for such great lectures, sir. They really help me in understanding things better.

[@brendawilliams8062]

This is personal note. I just want you to know I was able to progress. This meant the world to me. Thankyou.

[@asadoncasp]

A great legend that i have ever seen in my life. what a lovely man

[@seanki98]

sir, you are so amazing that today I had two conceptual questions that were not yet answered and I pondered a lot, and you answered both of them as clearly as humanly possible 9 minutes into the lecture :)

[@LeRainbow]

I just cannot fathom how a person can know so much. Mesmerizing and entertaining, even genius.

[@anandapatmanabhansu]

When i become a teacher i will try to show the world around them through experiments like you did and make them❤science.

[@jaydevzala7806]

Sir your teaching style is great thank you so much for this extra ordinary lecture.

[@icseselinaclass10]

Amazing lecture sir....really igniting the love for physics . thank you so much for such a beautiful explanation.

[@lecturesbywalterlewin.they9259]

+Megha Agarwal You are welcome

[@hrishikeshbharadwaj6189]

SIR,you really made me love physics even more.

[@lecturesbywalterlewin.they9259]

:)

[@Shiny_Mewtwo]

Professor, your usage of km ke constant is very good as beginners are always confused with these coefficients.

[@stauffap]

Great lecture! I love the derivations using maxwells equations. It might help me to understand the ultraviolet catastrophy, that surely fueled the search for something new like quantum mechanics.

[@lecturesbywalterlewin.they9259]

>>> It might help me to understand the ultraviolet catastrophy, >>> use googel

[@meikejune4009]

Phenomenal demonstrations. Physics is miraculous, light is the winner. Your polarisation sheets are very good for making the mystery visible.

[@lecturesbywalterlewin.they9259]

great birthday video Meike - thanx

[@meikejune4009]

@@lecturesbywalterlewin.they9259 Enjoy your day. I'm still in Pyjama:)

[@meikejune4009]

@@lecturesbywalterlewin.they9259 You are as young as me for the next half year when you turn the numbers. Walter, I'm with you with my thoughts.

[@lecturesbywalterlewin.they9259]

@@meikejune4009 still in pyjama?? it's 2 PM for you

[@lecturesbywalterlewin.they9259]

585

[@tashi2009]

1) 31:23min, Prof Lewin, what happened to miu zero? How it got cancelled? 2) In the first equation :when it comes to Electric field, Km is relative permittivity,if so, then why epsilon zero is not cancelled ? So grateful to you Sir, I have no enough words

[@lecturesbywalterlewin.they9259]

be more specific - I watched from 30:00 - 32:00 and see nothing worng

[@michaelgonzalez9058]

What happened to the 44

[@jonlasarte9690]

I wish I had a teacher like you

[@nehagoley]

very interesting and understanding lecture... thank you so much for this vedio lecture..

[@lecturesbywalterlewin.they9259]

:)

[@Jirayu.Kaewprateep]

You example about the reflection angle as use in fiber optic and also polarization we can use for light detector. I had comment about the light detector in your's polalization topic VDO. Another application except the car film is the oneway see through glass.

[@maqsoodahmad6378]

Tons of knowledge

[@Dhandawithsahooji]

Sir thank you for your teaching and i am seeing before exam

[@chrisallen9509]

This complements chapter 9 in Griffiths Electrodynamics nicely

[@nybble]

Once you wrote 4% on the board I let out a little yell... Cause that was the moment I was able to connect Feynman's QED lectures with this lecture...

[@aniketnandy3124]

Professor, I was just observing the interaction of lasers with mirrors and I observed that if a mirror is to make a virtual image, say a plane mirror, it actually makes both a virtual and a real image of the laser beam... but that shouldn't be possible, should it?

[@onkangijoshua1266]

Gifted Prof.

[@ratnajoybhowmick7889]

Sir, I have two questions. Suppose, while performing the young double slit experiment,instead of having double slits, we have a system of apertures which looks exactly like the Olympic rings. Surely it will not remain a double slit experiment. Then what will be the interference pattern and the diffraction pattern look like???? And my second question is that, like a Fresnel's bi-prism, if we have, say, a piece of diamond or an octahedral crystal , what will be the nature of the interference pattern?????

[@lecturesbywalterlewin.they9259]

1. if the circular slits are narrow enough, of course you will get diffraction. 2. ask google

[@munguis]

Prof. Walter, In problem 2 Pset 9 you mentioned that whenever we send a beam of light in a glass fiber the first bounce has the smallest angle of incidence, and that this angle will be larger for the following bounces. How do we know this? It does not seem very obvious

[@lecturesbywalterlewin.they9259]

it's due to the geometry

[@edisoncadena61]

DEAR PROFESSOR In what book can I see the boundary conditions of dialectrics?

[@lecturesbywalterlewin.they9259]

I used 8.02 Physics for Scientists & Engineers by Douglas C. Giancoli. Prentice Hall Third Edition ISBN 0-13-021517-18 8.03 Vibrations and Waves by Anthony French CRC Press ISBN 9780748744473 8.03 Electromagnetic Vibrations, Waves and Radiation by Bekefi and Barrett. The MIT Press ISBN 0-262-52047-8

[@enduraman1]

I don’t understand plane of incidence? When unpolarized light reflects off horizontal surface of water, instructor said plane of incidence is vertical and reflected light is polarized perpendicular to the plane of incidence - therefore, horizontal polarization. Why does unpolarized light from the sun have a vertical plane of incidence?

[@lecturesbywalterlewin.they9259]

The plane of incidence (AON) is the plane that contains the incident ray and the normal (ON, a line perpendicular to the surface) to the plane of the surface such that they intersect at the surface.

[@venkatakishore7276]

Hi Sir, At 29:50 , It was written tangential component of Electric field of media 1 is equal tangential component of Electric field of media 2. 1. How come both are equal when Kappa is different (i.e How tangential component of electric fields are equal at the boundary when media's are different) ? 2. Practically also Electric field of media 1 is equal tangential component of Electric field of media 2 ?

[@yogeshkumar-zv1ix]

sir can't we say.. that - sign also indicates that intensity of reflected wave is reduced by factor of 0.2 ?

[@anandapatmanabhansu]

Sir i studied Brewster law in school but didnot know it come from frenel equations.

[@The_Green_Man_OAP]

33:14 Finally I have found someone who can explain EM wave transmittance through materials properly! I am imagining that sheet 1 of electric charge that is normal to the overall propagation of the EM wave is turning against the next sheet 2, because wave 1 on surface 1 is inducing wave 2 on surface 2 that is orientated 90 degrees away from wave 1, then sheet 2 repeats the process with sheet 3 & so on... It's like when you drill into a wall and each layer spins up the next layer. - Is this a correct analogy? I see you have a diagram of the oscillating electric field plane alongside the oscillating magnetic field plane... Maybe the magnetic field oscillations are really the sum of (induced) components of retarded electric fields at different phases, locations and orientations to previous electric fields along the em wave? Makes me think of a twisting rope. c~√(K/I) ...⁉️ (Where: Torsional stiffness=K & Moment of Inertia=I.)

[@lecturesbywalterlewin.they9259]

great try again to solve problem 182 your answers so far were incorrect. Till now no one had the 2 correct answers. One person had 1 right,

[@rupeshgupta8929]

Respected Professor, Does light has mass? Can a magnet bend light? What decides the color of the light that we see?

[@lecturesbywalterlewin.they9259]

1. no 2. no 3. watch my lectures

[@babuj6949]

at 49.05 minutes in the lecture: i have a doubt regarding fresnel equation 1. after seeing lecture notes by some professors in google, i came to know that some are getting the fresnel equation 1 as you presented in the lecture. but some are getting like r(parallel)= (n2cos(theta1)-n1cos(theta2))/(n2cos(theta1)+n1cos(theta2)). which one is correct? why "-" sign is differing for different people?

[@stargazer7644]

babu J take a look at the lecture notes referenced in the dooblydoo

[@hellopomelo2]

Hi Prof, thanks for the lecture. Not sure if you would see this, but if it's convenient for you, I would like to ask a small question: so assuming we have a ray which is obliquely incident to a boundary, with a certain propagation direction . So is it right to say that I have to first find the direction of the E-field, break it down into its tangental and normal components relative to the plane of incidence, and from there on we can derive the fresnel equations from the boundary equations and snell's law? Thanks!

[@lecturesbywalterlewin.they9259]

I cannot add to the clarity of this lecture. Watch it again!

[@namesurn8590]

at 56:54 .so 4% of perpendicular component goes back and 4% of parallel component goes back, we say that 4% of light goes back. if for example 4% and 12% goes back, we say that (4+12)/2=8% of light goes back?

[@HimanshuSharma-xn6uc]

Prof. Is there video on Maxwell stress tensor and conservation of momentum and angular momentum of electrodynamics

[@lecturesbywalterlewin.they9259]

search online

[@flow8073]

Loved it!

[@PhongNguyen-ml4us]

Did he explained why reflection index of s-polarized always bigger than that of p-polarized (Rs>Rp)

[@mastipathsala912]

Sir what distance between mirror and object depends or not for see full image

[@lecturesbywalterlewin.they9259]

*dont be lazy* do your own ray tracing

[@anandapatmanabhansu]

Sir does the 100% reflected light (after critical angle) can be completely polarised.

[@lecturesbywalterlewin.they9259]

In the case of the Brewster angle, the reflected light is 100% linearly polarized but 100% of the light is *not* reflected. In the case of Rayleigh scatering at angles of 90 degrees the light is 100% linearly polarized but again *not* 100% of the light is scattered.

[@anandapatmanabhansu]

@@lecturesbywalterlewin.they9259 Thankyou sir❤

[@anandapatmanabhansu]

Sir can after critical angle, the brewsters angle can be achieved.

[@xavierzamora6455]

Can the equivalent Fresnel coefficients still be valid even if the imaginari part of the refractive index is different? Because here the imaginary part of both n1 and n2 is i00 (48:35)

[@alexandgarciacalle]

dumb question: why doesnt he write or use the vertical component of the B(transmitted) vector in minute 40:48? Thanks

[@brendawilliams8062]

Thankyou.

[@zakirshah5060]

Hello , hello hello , where I can find your all lectures?

[@erikhicks6184]

Looking forward to 176 demo

[@ratnajoybhowmick7889]

Sir, we know that yellow light is a monochromatic light wave, and when white light is incident on a prism it produces "vibgYor". However, from color triangle we know that "Red light+Green light=Yellow light"....so, does yellow light remained "monochromatic"? If yes, then ,is their any difference between these two yellow lights?

[@lecturesbywalterlewin.they9259]

use google

[@lecturesbywalterlewin.they9259]

google "monochromatic" light and use google for more information

[@sohamsen5967]

Professor, i was stuck with a question and it would be great if you could help me out with it: Question was: A trough is filled with water upto a height H, a plane mirror is placed below the trough. A fish is there at a distance H/2 from the bottom. If an observer looks normally at this arrangement from a height H (measured from the air water interface), how many images will the observer see and at what positions. I could figure out one image (formed due to upward shift: the apparent position of the fish) But I couldn’t understand the image formation due to the mirror. It would be really helpful if u could explain it to me. Thank you

[@lecturesbywalterlewin.they9259]

I do not solve problems for viewers - I teach Physics. Besides I do not understand the problem.

[@sohamsen5967]

I was not requesting for a solution i wanted to understand the physics behind the image formation. Thanks a lot anyway.

[@lecturesbywalterlewin.they9259]

@@sohamsen5967 I can't teach you optics in 3 minutes - I suggest you use google

[@dailyfacts581]

Excellent

[@lecturesbywalterlewin.they9259]

Thanks

[@ratnajoybhowmick7889]

Sir, why our brain is not capable to detect the magnetic field, but can detect electric field?.....(I am sorry if it is not a physics related question, but I want to know)

[@suhanahussain7]

Have u read about "Magnetoreception"?

[@rjong1126]

Dear Professor, I have a question when I reach t=1:09:42. According to your lecture 16 at t=57:32 you said that the electric field which parallel to the teeth of the comb should not pass through the comb. Then , why shouldn't the glass manufacturer choose the 1st option of the given choices. As you said the reflected light from the surroundings would be perpendicular to the plane of incidence, then in order to get rid of them shouldn't we choose the glasses with pattern given in the 1st option? Thank you

[@lecturesbywalterlewin.they9259]

your questions are unclear

[@rjong1126]

@@lecturesbywalterlewin.they9259 Maybe I should ask: Referring to the options of the patterns for the glasses which you gave in the lecture, does the pattern indicate the direction of polarization of the glasses or the strips on the glasses( which has the same function as the teeth of the ''comb'' you mentioned in your lecture 16 57:32)? Thank you

[@lecturesbywalterlewin.they9259]

polarizers have long stressed out molecules - use google to read up on them. The process by which they produce polarized light is different from the absorption of radar when the polarized E field of the radar is parallel to the teeth of the metal comb.

[@rjong1126]

@@lecturesbywalterlewin.they9259 so you were talking about the direction of the polarization ( the direction which is allowed to pass through the glasses) when you formulated the questions ( glasses manufacturer), right?

[@Originalimoc]

The answer is the question sentence start exactly at 1:08:09.

[@MichalShimanovich]

Hello Professor Lewin, As far as I understand, Fresnel equations describe the macro-world, but do not describe the micro-world (and they are mostly mathematical explanation of this phenomenon). By micro-world I mean the inner material (molecular) interactions with light. What I'm interested in is what exactly, in the dielectric material surface causes this "polarization based" selective reflection of light? (My first thought was that the dipoles in material 2 arrange that way that, for example, at Brewster angle they radiate no light in the reflected direction....).

[@lecturesbywalterlewin.they9259]

:)

[@beletewube]

What about the type of mirror,in most cases the refraction happens not in the plain mirrors.

[@lecturesbywalterlewin.they9259]

question unclear, use Quora

[@rahuld.khileri1663]

Sir where can I get all of your physics lectures

[@lecturesbywalterlewin.they9259]

they are all in this channel

[@stasbucik6234]

How does he draw the dotted lines??

[@mastipathsala912]

Sir if a man wants to see their full image then what will be minimum length of mirror

[@lecturesbywalterlewin.they9259]

flat mirror half of your length

[@yashaswiniv.2774]

Can someone tell me why there is no light intensity loss when he twisted the fibres? I’m confused. Shouldn’t the angle change when you bend and there is loss? Also what is bending radius? Unrelated to the twisting he did?

[@lecturesbywalterlewin.they9259]

there is zero loss as long as everywhere in the fiber sin(i) > 1/n (n is the index of refraction of the fiber).

[@yashaswiniv.2774]

@@lecturesbywalterlewin.they9259 Thank you so much for your reply. But my question is why is that (i) doesn't change even while twisting! I am not able to visualize this.

[@lecturesbywalterlewin.they9259]

@@yashaswiniv.2774 *there is zero loss as long as everywhere in the fiber sin(i) > 1/n (n is the index of refraction of the fiber).* THINKKKKKKK

[@bilalhussein9730]

@@yashaswiniv.2774 You do actually get losses in bent structures but not for the reason you are thinking of. If you draw the situation for, say, a ring and use the law of reflection you'll see that the professor's statement is correct if you ignore more advanced physics. But in reality the wire radiates away energy because of the bend. With the introduction of the bend the wire is no longer translationally invariant so the bound eigenmodes of the formerly straight wire, which are the ones transporting energy, are no longer eigenmodes but must have a coupling with a radiation field. Now you might ask. Well if the previously straight and now circumferential modes aren't lossless are there any lossless modes of the bent waveguide? Yes there are. If you solve Maxwell's equations for a cyindrically symmetric system but they turn out to have two interesting features: they are radial and DON'T transfer any power.

[@Thats12789]

You are looking like IMRAN KHAN in the THUMBNAIL

[@kiranbhat4902]

Walter sir why do we see an inverted image of our face on a spoon(concave side) if real images can be only obtained on a screen????

[@lecturesbywalterlewin.they9259]

high school optics - use google

[@kiranbhat4902]

Walter sir I am a 10th grader I have learnt high school optics this is my only doubt.... Could u please clear it.... I didnt find it in Google.....

[@lecturesbywalterlewin.they9259]

@@kiranbhat4902 www.physicsclassroom.com/class/refln/Lesson-3/Image-Characteristics-for-Concave-Mirrors

[@kiranbhat4902]

@@lecturesbywalterlewin.they9259 Thank you sir but my question was how can a real inverted image be obtained on a spoon itself???(as real images can be obtained on a screen only )

[@lecturesbywalterlewin.they9259]

metal spoon is a mirror

[@Gaurav-vj6yo]

Sir can a beam of light be reflected by another beam of light ??

[@someshskanda5029]

Sir, you have taken Theta 1 = Theta 3..but in snell`s eq you have written n1sin theta1 = n1sin theta2...why?? is that correct...??

[@lecturesbywalterlewin.they9259]

how many minutes into the lecture?

[@someshskanda5029]

8:40 min, Sir...thanks for reply..

[@lecturesbywalterlewin.they9259]

Theta 1 = Theta 3 correct - reflection n1sin theta1 = n1sin theta2 INCORRECT *refraction n1sin theta1 = n2sin theta2*

[@someshskanda5029]

Lectures by Walter Lewin. They will make you ♥ Physics. ty...sir....you really help me learn physics....ty

[@jawad8727]

43:00 I'm stuck in Ei+Er=Et .. why not Ei = Er+Et ... Incident = reflected + transmitted

[@yaskynemma9220]

Because the boundry condition is E1tan=E2tan from the aplication of faradays Law that he did, it seems strange because we want to see the conservation of what enters and exits the interphase, but this compares the E propagating and not E "penetrating" I think

[@cenarvd]

Brewster Angle 58:20

[@someshskanda5029]

sir, at 8:23....page 472..which book is that?? can you name it plzz..

[@someshskanda5029]

8:20 to be precise..

[@lecturesbywalterlewin.they9259]

8.03 Vibrations and Waves by Anthony French CRC Press ISBN 9780748744473 8.03 Electromagnetic Vibrations, Waves and Radiation by Bekefi and Barrett. The MIT Press ISBN 0-262-52047-8

[@someshskanda5029]

Ty, sir....

[@someshskanda5029]

ISBN 0-262-52047-8 sir can I download it online...if yeas can you provide me the link..and thank you sir..

[@lecturesbywalterlewin.they9259]

I don't believe this can be downloaded.

[@Sanjay-zt6bg]

1:10:46 Nice one

[@namesurn8590]

at 1:18:37, why when light refleted it loses polarization?

[@bilalhussein9730]

Some of the light is diffusely reflected and therefore unpolarized.

[@राजेशकुमार-ढ8ख7ल]

SIR what that inverted triangle denote in 25:06 8.03 lecture 18

[@lecturesbywalterlewin.they9259]

it's de DEL vector www.google.com/search?safe=active&source=hp&ei=AIXbXpu2LcquytMP6q-PyAM&q=del+vector&oq=del+vector&gs_lcp=CgZwc3ktYWIQAzICCAAyAggAMgIIADICCAAyAggAMgYIABAWEB4yBggAEBYQHjIGCAAQFhAeMgYIABAWEB4yBggAEBYQHjoFCAAQgwE6BQgAELEDOgQIABAKUL9WWJpkYMtmaAFwAHgAgAFLiAGqBZIBAjEwmAEAoAEBqgEHZ3dzLXdperABALgBAg&sclient=psy-ab&ved=0ahUKEwibttrRke3pAhVKl3IEHerXAzkQ4dUDCAk&uact=5

[@राजेशकुमार-ढ8ख7ल]

@@lecturesbywalterlewin.they9259 thanks sir

[@राजेशकुमार-ढ8ख7ल]

@@lecturesbywalterlewin.they9259 sir i am a student of 12 just pass can these lectures are helpful to me

[@राजेशकुमार-ढ8ख7ल]

sir when i started doing numricals and when the concept of another chapter comes in between the solution i in most of time unable to understand which concept is used there so sir i want an advice from you that how can i improve that tecnique sir plzzzzzzzzzzzzzzzzzzzzzzzz

[@ahmedmaher-uu4je]

why incident angle betwen incident ray and normal not betwen incident ray and reflection surface

[@carultch]

Because the normal is by definition, perpendicular to the surface, and that is the convention used in optics for the way to measure angle of incidence, angle of reflection, and angle of refraction. You could opt to define angle of incidence for reflection to be the angle from the reflection surface to the incident ray, and it would indeed equal the angle from the reflection surface to the reflected ray on the other side. However, this convention wouldn't work with Snell's law, the way Snell wrote it. We'd have to switch from sines to cosines, to work with angles from the surface, instead of angles from the normal.

[@felixflorack5838]

Wow

[@yajatagarwal5809]

Dear sir please help me to understand why the frequency of E field is same in both the mediums at duration 6:52

[@lecturesbywalterlewin.they9259]

if light changes media with different index of refraction, the wave length will change but the freq cannot change. That's also the case with mechanical strings of different speeds of propagation, wavelength changes but freq cannot change *THINKKKKK why that is* Einstein's SR: The energy of a photon is hf, f is the frequency. When a photon goes from one medium into another its energy does not change, thus its freq does not change.

[@yajatagarwal5809]

@@lecturesbywalterlewin.they9259 But sir can the photons interact with the media and maybe lose energy in the form of collisions which can also correspond to change in its initial angle. Correct me sir if am wrong anywhere please

[@lecturesbywalterlewin.they9259]

@@yajatagarwal5809 Go out in the sun, many photons will be absorbed by your face. That can heat your face and your face wil then radiate EM radaition as more or less black body radiation, but that had nothing to do with Snell's Law. If light goes from one transparent medium (air) into clear water, it's speed changed dramatically but its frequency does not change. But if the water is not transparent, light can be absorbed.

[@yajatagarwal5809]

Thanks a lot sir for your time. Unambiguously you are the best teacher i have ever had ❤️

[@jaydevzala7806]

Sir please tell me where I can find book becumferenberet.

[@lecturesbywalterlewin.they9259]

I have no idea

[@stefano0912]

1:11:50 sus