8.03 - Lect 20 - Interference, Huygen's Principle, Thin Films, Double Slit Demo

[@divyajothibalasubramanian8042]

This lecture was a huge relief to all the days of my struggle in understanding! Thank you, professor. Wishing you health and happiness from India.

[@sagnikbiswas2227]

Hey are u preparing for JEE or NEET?

[@chitramoorthy3566]

Love from Tamil Nadu, India ❤️🔥

[@jamilurrahman6360]

M from Assam

[@hollywoodhub8580]

Nanum thala

[@blackinsane2239]

I am Also Bro💖

[@prasannahd]

🥰🥰🥰same

[@pushendrapatel5500]

Me too

[@ImtaeZ]

I was confusing about phase shift before thanks Professor Lewin to make me understand it!!🎉

[@lecturesbywalterlewin.they9259]

interestedvideos.com/physics-youtube-channels/

[@sujay8181]

Loved ur lecture as always sir. Is there any way of performing the double slit exp at home? If yes can u please tell how to do it ?

[@lecturesbywalterlewin.they9259]

yes very easy - Take a piece of paper and a razor blade. Make 2 cuts with the razor blade in the paper close together (less than 1 mm apart). Hold the 2 cuts in front of one eye and look at a bright light - you will see interference,

[@sujay8181]

Thank u very much sir.

[@sauravsaw501]

@@lecturesbywalterlewin.they9259 That was amazing!!! wow!!

[@cherryhuhhh]

Sir , i loved your lecture . Your lectures are very helpful. Thank you so much From India

[@miazain8534]

Sir I'm ur biggest fan ....u make me love physics ❤️❤️...ur lectures are really helpful...

[@lecturesbywalterlewin.they9259]

Glad to hear that

[@kajalandgovindgupta7409]

Thank you so much sir for your wonderful explaination, my all doubts have cleared Love your teaching style 😊

[@farooq8897]

We love your lectures. Can we expect a few lectures on quantum mechanics and relativity from you, sir? From India

[@lecturesbywalterlewin.they9259]

here they are: kzbin.info/www/bejne/oouWk4OLgNRosMU kzbin.info/www/bejne/h3qUZIZmadxrmq8

[@preetamdeshwal5135]

You are the only who replies. 💢❤️

[@NamanRawat-g9c]

In India We Generally Used To Study All This In High School And You Are Teaching This To University Students 😂

[@lecturesbywalterlewin.they9259]

Keep in mind that MIT is only a very mediocre University. That's why only 100 people who studied at MIT or who lectured there received a Nobel Prize. *That's very embarrassing.* Luckily you got a much better education.

[@NamanRawat-g9c]

@@lecturesbywalterlewin.they9259Yes You Are Right But That Time Has Now Gone And Now You Can See Many Survey Articles That Indian Engineering Institutes Are Much Better In Producing Better Skilled Engineer Then MIT That's Why Indian Engineers Are At Leading Post Of Many Big Tech Firms And Same Also In Research Field

[@maqsoodahmad6378]

Greatest sir of all time

[@rajdeeproy8182]

Thank you, very much ,sir. Your lectures are extremely beneficial for students like me who prefer to study on their own.

[@lecturesbywalterlewin.they9259]

It's my pleasure

[@Radha_dass]

Great sir,loved your way of teaching. I was not getting it since long but now I am clear of the concept. Can you please tell me reason behind polarisation on scattering.

[@lecturesbywalterlewin.they9259]

I explain that in my 8.03 lectures (Fresnel Eqs).

[@jayantachoudhury4397]

Sir... U r genius.. Tnq so much for these amazing lectures.. But sir I've a doubt.. What will happen if the width of the slit is less than the wavelength of the light used in the double slit experiment?.. Please reply me sir... I will be highly obliged.. Tnq so much sir.. ❤️❤️

[@lecturesbywalterlewin.they9259]

That's a good question. Let's make the wavelength much larger than the width of each slit. Do it with radio waves. No radio waves will go through any slit which is only 0.1 mm wide.

[@jayantachoudhury4397]

@@lecturesbywalterlewin.they9259 Tnq so much proffesor for give me your valuable time... Sir may you prefer some best books for physics from which you were teaching regularly?? Please sir..

[@virenderbhardwaj3137]

4:50 thank god ,it has been said here, wikipedia also tells this but many teachers don't say it which confuses me , I am in high school currently and huygens principle just don't makes sense but it gives the right answers so I should just memorise it as it is at my level otherwise I don't think there is any other way to understand these things at my level

[@nirbhaythakkar9931]

Love From Gujarat, Indian

[@fernandomarsal6268]

You are a real Master!

[@marufhasan9831]

If Huygen's principle gives the right answer for the wrong reason, then what is the right reason?

[@imaginary_pies]

I wish these lesson were available 20 years ago when I dropped uni

[@chrimony]

@1:10:00: Maybe the center is higher, contour-wise, which makes the oil there thinner?

[@nikhilsaharan7470]

How do you draw those dotted lines in such a perfect way

[@harmoniumcover5369]

Put the chalk in the direction your hands moves while drawing with the angle of 135°

[@medinebagirova2468]

dear walter lewin you are my favorite teacher♥️

[@surendrakverma555]

Excellent lecture Sir. Thanks and Regards 🙏🙏🙏🙏🙏🙏🙏🙏

[@lecturesbywalterlewin.they9259]

Most welcome

[@rajeenasayyedali3963]

Professor... At 30.10, is it the experimentally observed interference pattern of white light that you showed on the slide...??? I'm sorry if I'm wrong but I thought it would come in the form of coloured bands and not as dark and bright bands...

[@PraveenKumar-kb3lq]

Sir I have query regarding a phenomenan that I come across many time When ever I pick out my phone in sunny day then I find the in black display that rays which are coming out from the sun seems to be colorful but the sun looks white could explain me this happen

[@lecturesbywalterlewin.they9259]

refraction

[@mtabernig]

waves are never parallel. they always diverge. Only lasers have quasi parallel, Pure parallel do not exist. Hence the term broadcast.

[@xiip2-2436]

Why is the sign convention used in the derivation of the lens formula and yet used again when it is applied in numerical problems? Won't the whole idea of sign convention be eliminated if it is used twice?

[@xiip2-2436]

Plz tell plz plz Plz Plz Plz

[@lecturesbywalterlewin.they9259]

use google

[@xiip2-2436]

@@lecturesbywalterlewin.they9259 Google doesn't has answer(appropriate)

[@lecturesbywalterlewin.they9259]

google: "The sign convention for lenses is similar to that for mirrors. Again, take the side of the lens where the object is to be the positive side. ... Similarly, a converging lens always has a positive f, and a diverging lens has a negative f. The signs associated with magnification also work the same way for lenses and mirrors."

[@pargatsingh594]

@@lecturesbywalterlewin.they9259 but professor we already used them while deriving the equation. Why should we use them again?

[@Merilix2]

Huygens principe drives me crazy. Why? Coud be an answer where the energy of red-shifted light in a expanding universe is. Huygens sayd, each point in the path of a traveling wave can be seen like another point-source of a wave. So far so good. Point sources besides each other interfer and form a wave front, waves passing slits form a interferrence pattern, waves traveling backwards cancel each other out. So far so good. But what happens if the media expands while waves travel through it? Waves traveling backwards cant cancel each other completely out because of phase shift so there is something left. Well, the universe expands, light traveling through it gets stretched and red-shiftet. But where is the energy? Some physicists say: the energy is just lost. They say conservation laws dont hold in a expanding universe. But I have the imprssion if you apply Huygens principle you get the answer. Very tiny parts of the propagating light waves are just reflected (scattered) at the expanding vacuum because those parts are not canceled out completely. Tiny enough to become immeasurable but enough to tell where the lost energy is? Is there something wrong with that idea?

[@poojaubanshi8782]

Why did you choose Electric vector not the magnetic vector as in case of electromagnetic wave both are perpendicular to eachother??

[@lecturesbywalterlewin.they9259]

you can choose one or the other.

[@rudilapa6569]

This is an awesome lecture + demonstration. Thank you.

[@Jirayu.Kaewprateep]

Thank you, I like the sentence about "I will instruct this course in the wrong way to the right results". It feel like are you joining the dark force or the light force. [Monochromatic light]

[@sweetysingh7113]

why newton ring called film of equal thickness thank you sir for wonderful videos

[@lecturesbywalterlewin.they9259]

who called it "equal thickness"? The thickness of colorful oil is not the same everywhere. If that were the case, there would not be so many different colors.

[@kuldeepsinghrathore5290]

Thank you❤

[@lecturesbywalterlewin.they9259]

You're welcome 😊

[@greensponge6538]

Professor, could you explain how to show that at greater thicknesses, many different wavelengths start to constructively interfere? This problem has me stumped

[@rajeshrisadulwad2001]

Sir, I can now pronounce Huygen correctly and I guarantee you that I am not a Dutch.It is a great video . Can you suggest me some books on physics.

[@ChessAndNotCheckers]

So can I mate...The Dutch 'g' is an astonishing thing Edit:To be fair, the name 'huygen' itself is astonishingly beautiful too

[@sidhu.nishant]

If we can obtain dark fringe with help of two light is the converse also true means can we obtain bright fringe with no light

[@apollolasky7079]

1.6 million subscribers yet only 85k views in 8 years. Ignorant youtube.

[@thienthanhtranoan1011]

Teacher, can we take a photos together¿

[@preranadash9640]

Sir is the interference pattern is produced on the thin film or in between the medium or in the air medium?

[@lecturesbywalterlewin.they9259]

I explain this in detail in my lecture

[@samuelr879]

Sir ,I have a doubt. What will happen if the width of the slit is less than the wavelength of light used in the double slit experiment?

[@lecturesbywalterlewin.they9259]

good question - if N photons reach the double slit per sec a major fraction of N would not emerge from the slit but would either be reflected or absorbed by the slit.

[@samuelr879]

@@lecturesbywalterlewin.they9259 thank you sir

[@bhend1]

One question . When we use white light in YDSE which colour appears close to Central Maxima?

[@potato733]

Blue because it has a shorter wavelength. The distance of the secondary Maxima from the central is proportional to λ. (d sin θ =nλ)

[@bhend1]

@@potato733 But if we look b/w central maxima and secondary maxima there is a First Minima. So due to smaller wavelength violet would have its Minima and if its a minima for violet, violet would not appear and hence shouldn't it be red?

[@atulchandapa4018]

White

[@shama.saravu]

@@bhend1 we will see 🔮Violet colour near to central maxima.... bcz it has shorter wavelength....☺

[@shama.saravu]

@@potato733 No..... We see Blue in scattering phenomena...😊

[@pushpamsingh3870]

Thank you, professor. Why are there so many adds during the lecture?

[@lecturesbywalterlewin.they9259]

there is supposed to be ONLY 1 at the beginning at that one can be by-passed,

[@satyamparija7614]

Found 8.03x lecture 20, how to do doubt sheet experiment at home, please tell

[@chudamanikalele2825]

Sir are these wavefronts way of representing light waves or anything in reality .Sir because in case of water we say wavefronts are Produced when a particle vibrates.Sir if you get time plz respond

[@saurabhpandey4153]

sir,from where i get pdf file,assignment of your al lecturesl

[@aniketeuler6443]

From the description of the respective lectures 👍

[@lecturesbywalterlewin.they9259]

they are below the video (after you open the video) as PDF files. Alternatively open my Playlists "Assignments, Exams, Solutions and Lecture Notes." There is one for each of my 3 MIT course: 8.01, 8.02 and 8.03

[@sandrocavali9810]

Maestro!!

[@davidh2049]

@Lectures by Walter Lewin. They will make you ♥ Physics. With the thin film experiement, we can see a large dark area because of destructive interference. The question is where does the energy go? Energy cannot should not be destroyed by the interference, but what happen to the energy ?

[@lecturesbywalterlewin.they9259]

take a simple double slit or silngle slit diffraction pattern. Look at the curves of intensity vs angle. The constructive interference intensity peaks are high - that's where the energy of the dark areas are.

[@davidh2049]

@@lecturesbywalterlewin.they9259 Thank you for your reply. In the single slit and double slit cases, there are constructive interference as well as destructive interference. But in the case of thin films cases, if the films are very uniform, then all the lights can be destructive interference without any constructive interference. That means the light intensity is 0 (or very low)all over the area. In that case, where does the energy go?

[@saumya942]

Professor, can you please make a video about Newton's Rings?

[@lecturesbywalterlewin.they9259]

use google

[@saumya942]

Lectures by Walter Lewin. They will make you ♥ Physics. You're a better teacher than Google 😁

[@harshameti1066]

At 6:42, Is the hyperbolic surface a wave front?

[@debayankundu1344]

Thank you Sir.

[@rajeshrisadulwad2001]

Sir, I have a question . In solar cells , a silicon solar cell [Refractive index =3.5] is coated with a thin film of Silicon Monoxide [Refractive Index =1.45] to minimize reflective losses from the surface. Determine the thickness of SiO that produces the least reflection at a wavelength of 550 nm , near the center of the visible spectrum.

[@lecturesbywalterlewin.they9259]

I do not solve problems of viewers, I teach Physics. I cover the physics in my lectures - or use google.

[@rajeshrisadulwad2001]

Thank you for suggestion sir .

[@heramb575]

Professor why doesn't intensity fall off with inverse square of distance in YDSE like in a usual light source/wavefront ?

[@lecturesbywalterlewin.they9259]

use google - also google Lasers

[@rajeenasayyedali3963]

Sir... At 47.40,you told u added the (pie) Because at the 1st interface n1n3... Can u please explain the reason... I couldn't make it out from Google or other sources.

[@rorythewriter]

The sign of the reflection coefficient indicates if the change of medium flips the wave ( - ) or not ( + ). The first change does (r = -0.2), the second doesn't (r = +0.2)

[@kindleright2846]

Sir, when wave is reflected from denser medium the reflected wave leads in phase or lags in phase. Is there any way to derive an equation

[@lecturesbywalterlewin.they9259]

I cover all this in my lectures. I cannot add to that.

[@abdullahahmad2474]

Sir interference is a very wide topic, were can I get lectures about detailed Topics of interference like newton's rings,interferometer,etc?

[@lecturesbywalterlewin.they9259]

I cover interference in my 8.02 and 8.03 lectures.There are also many on the web. Try the one by Feynman kzbin.info/www/bejne/aJ6snGatd693nac

[@abdullahahmad2474]

Lectures by Walter Lewin. They will make you ♥ Physics. Sir that Richard feynman lectures are really great... But is there any website/page that covers detailed topics of graduation physics? ...

[@lecturesbywalterlewin.they9259]

use google

[@physics110]

in the thin film part, what are the sources of light that give interference?? you have to have two sources of light in order to have interference , right??

[@lecturesbywalterlewin.they9259]

sun light

[@arifullah7395]

sir in case of interfernce due to thin film if we use white light than after reflection the two waves interfre destructively and rest of the colours are a mixture of constructive interfrnce nd as reult we observe the coloured pattern?....sir is this is right explanation fr using White light instead of monochromatic light in case of Interfernce due to thin Film?????

[@lecturesbywalterlewin.they9259]

watch the lecture again

[@arifullah7395]

Sir according to Netwon Light shows particle naturr but the rings formed is closely resembles with wave than is it the contrardiction

[@lecturesbywalterlewin.they9259]

use google - light is both (photons and waves)

[@adamlantos2319]

Professor, why do we call them thin films? Is there anything wrong with thick films? In thick films, the phase difference can also be such that it produces constructive or destructive interference, so why don't we just say "film interference"?

[@lecturesbywalterlewin.they9259]

+adam lands Interference happens at lambda = 2 * n * t / m for normal incident light where n is the index of refraction, t is the thickness and m is an integer from 1 up. This means the difference between two adjacent lambda's that show interference (i.e. lambda where m=1 and m=2) is largest with low m and goes to 0 as m goes to infinity. For a relatively thick film (with n=1.5) of 1 cm lambda_1 (lambda where m=1) is 3 cm. lambda_2 is 1.5 cm and so on. For visible light, which has wavelengths of 400 nm to 800 nm, m is of the order of 60000. Since the difference in wavelength between lambda_60000 and lambda_60001 is so small there is no noticeable difference between these two wavelengths and you will not see interference as "all" wavelength in the range 400 nm to 800 nm overlap. .

[@adamlantos2319]

Lectures by Walter Lewin. They will make you ♥ Physics. Thank you professor!

[@harshameti1066]

Sir , Is the hyperbolic surface a wavefront?

[@lecturesbywalterlewin.they9259]

use google

[@smitaganguly5349]

Sir, will there be interference in a thick film if I use a monochromatic source of light?

[@lecturesbywalterlewin.they9259]

yes, there will be interference, however, you may only be able to see it if the film is very thin.

[@smitaganguly5349]

thank you :)

[@studyingchannel2328]

Cos2 delta meaning intensity . Am I right

[@shinglau9323]

why we get only one pi//there is no phase difference of pi of the reflected light (between oil n2 and air n3)// in the film interference. Would you mind elaborating a bit, please?

[@lecturesbywalterlewin.they9259]

+Shing Lau how many minutes in the video?

[@shinglau9323]

+Lectures by Walter Lewin. They will make you ♥ Physics. Thanks for reply! It was about 47:20

[@lecturesbywalterlewin.they9259]

+Shing Lau n1-n2 0 minus =>phase change pi. + => no phase change.

[@shinglau9323]

+Lectures by Walter Lewin. They will make you ♥ Physics. Thanks for replying! that's exactly what confusing me, I don't understand why. The proof seems lie in the boundary conditions of EM waves. ha probably I should not have skipped the lessons before in this course.

[@jashan3024]

sir, how did you derive this formula for refractivity...? r(A)=(n1-n2)/(n1+n2) i could not find it on net anywhere.. neither in my text books

[@lecturesbywalterlewin.they9259]

how many minutes into my lecture?

[@jashan3024]

@@lecturesbywalterlewin.they9259 41:35

[@lecturesbywalterlewin.they9259]

This is lecture 20. I covered this in detail in Lecture 18. kzbin.info/www/bejne/lXWUq2mqZ99kbJo

[@jashan3024]

Lectures by Walter Lewin. They will make you ♥ Physics. Thanks a lot!!

[@preranadash9640]

Sir then why the edges are white in colour in the last photo

[@lecturesbywalterlewin.they9259]

how many minutes into the lecture?

[@kalyanreddyn1124]

at 18:46 what does "n" belong to

[@ANMOLSHARMA-is5yw]

Why is it that the phase angle be integral in constructive interference and fractional in destructive interference?

[@lecturesbywalterlewin.they9259]

The basic idea of interference is that the waves are IN PHASE for constructive and 180 degrees out of phase for destructive interference.

[@sweetysingh7113]

in some book i have read newtons ring also called fringes of equal thickness why it is called so ? is this wrong or right

[@lecturesbywalterlewin.they9259]

they probably define a fringe as an area ring (with certain width) which has only ONE color. That ring has then ONLY thickness. Different colors, different thickness.

[@sweetysingh7113]

but sir even for one color as we go from central dark ring diffrence b/w nth and n-1 th ring get so close then how can we say fringe of equal thickness even for one color thanks for reply and giving your precious time

[@lecturesbywalterlewin.they9259]

encyclopedia2.thefreedictionary.com/Fringes+of+Equal+Thickness

[@miranaangel9622]

Hi professor, I have a question about the thin film interference. In your example, r=(n1-n2)/(n1+n2)=0.2, so the reflected light is 4% of the original, and the transmitted light should be 96% of the original. However, if we calculate t=(2n1)/(n1+n2), we will get 0.8, and its square is 64%, not 96%. Did I misunderstand anything here?

[@lecturesbywalterlewin.they9259]

telescopes lenses lose 4% of the light intensity. Use google.

[@namesurn8590]

did you overlook that v is diffrent in other medium? you can;t just square 0.8. Watch lecture 18 at 57:34

[@pushpendrakapri3434]

professor suppose as in your experiment if light that reaches to second slit is of phase pi/4 ahead of light that reach first slit then where will central maximum shift Is it above the center line or below it.

[@lecturesbywalterlewin.they9259]

If you determine through which slit a photon goes, then there will be no interference. This is the GREAT mystery of QM

[@pushpendrakapri3434]

professor i am not determining through which slit photon goes I am considering that if there is a phase difference between the light that reaches two slits then where will central maximum shift. Is it above the central line or below it?

[@lecturesbywalterlewin.they9259]

your thinking is wrong. You think Newtonian that is not allowed

[@nadinemouchet6484]

Hi Pushpendra, If I may, I will take a stab at your question here, with a more lengthy answer. You ask how the central area (wave) of light in the interference effect, and (I'm guessing) the whole pattern, is affected if you know there is a phase difference between the photons going through the top and bottom slits. What you need to know is that the wave function of light is determined by a probability distribution of different frequencies at which the photon can be found when observed. This means there is an uncertainty as to exactly what the frequency is. Fourier analysis describes how this is equivalent in the case of position, and how uncertainty in frequency is linked to uncertainty in position. When position is certain, frequency is less certain and vice-versa. Now regarding a phase difference among photons, if we take for our double-slit experiment a light composed of a single frequency, and if we consider in this light there are phase differences between photons, the frequency of the light's wave function will be unchanged (unless destructive interference completely kills the wave function, which kills the experiment entirely). This means having different phases of light through the slits maintains the same uncertainty of position we already had in the video (because the uncertainty of the frequency of the light remains unchanged). This uncertainty of position is what creates the wavepacket-like pattern you see in red on Mr. Lewin's screen. Indeed this represents the probability distribution of the position of the photons passing through the slits. For more on the uncertainty principle, see MIT's OCW 8.04 course, available on KZbin. I hope this helped. :- )

[@anmolmor30]

I guess I understand what you are trying to ask @pushpendra , given that the wave reaching the second slit (say the bottom one) already has an additional phase difference as compared to the first one, then the wave from the first one will have to cover an extra path to compensate for its lag thus shifting the central fringe (i.e. zero path diff fringe) and hence the entire pattern towards the second slit i.e. downwards !

[@poojaubanshi8782]

Your hair are way smaller than usual

[@leidarmaring7430]

Hello sir I'm a student I want to begin my career as a physics professor if I need any help I may contact at any time pls b on online

[@Maths_Magic_]

do you have walter lewin sir number

[@himangimahajan3631]

We pronounce it as HiiGun

[@b30wulffz]

it seems like u did over 5000 of experiments on physics in ur career

[@lecturesbywalterlewin.they9259]

yes

[@b30wulffz]

glad to hear from you, Will try to get more knowledge from u as much as possible. Thanks a lot😊

[@sauravsuman3017]

awesome

[@lecturesbywalterlewin.they9259]

+Saurav Suman :) :) :)

[@sagniksaha7359]

Sir in the single slit diffraction experiment, how does the width of the slit affect the intensity of the central Maxima? For instance, if we double the width of the slit, how does it affect the intensity of the central Maxima?

[@lecturesbywalterlewin.they9259]

to first order, within certain limits which depend on the ratio of the slit width and the wavelength of the light, the intensity of the central max will scale linearly with the slit width.

[@sagniksaha7359]

@@lecturesbywalterlewin.they9259 Thank you sir

[@tkzrii102]

wow!

[@techniclg.g.8906]

India