8.02x - Lect 34 - Diffraction, Gratings, Resolving Power, Angular Resolution

Diffraction Gratings, Resolving Power, Single-Slit Diffraction, Angular Resolution, Human Eye - Telescopes
Assignments Lecture 33, 34, 35, 36: freepdfhosting.com/33429fbcb9.pdf
Solutions Lecture 33, 34, 35, 36: freepdfhosting.com/1d839ac5ee.pdf
Optional Assignments Lecture 33, 34, 35, 36: freepdfhosting.com/b12020bee7.pdf
Optional Solutions Lecture 33, 34, 35, 36: freepdfhosting.com/de2e9ba7f2.pdf

[@lecturesbywalterlewin.they9259]

This website contains all my 94 course lectures (8.01, 8.02 and 8.03) with improved resolution. They also include all my homework problem sets, my exams and the solutions. Also included are lecture notes and 143 short videos in which I discuss basic problems. ENJOY!

[@JarredDavidson]

Professor Lewin, thank you for making these lectures freely available. They are incredible. I am a physics student and I find your lectures to be the perfect supplement to my course. You are an amazing teacher! THANK YOU!

[@lecturesbywalterlewin.they9259]

You're very welcome!

[@jayesh6049]

Sir, lots of love from INDIA ❤ thank you for giving such amazing lecture it help us a lot in understanding such difficult topic . thank you sir ,

[@lecturesbywalterlewin.they9259]

You are most welcome

[@ourdestinysee]

your lectures are a blessing prof!! they indeed made me fall in love with physics. lot's of love and respect to you from across the globe 🙇

[@lecturesbywalterlewin.they9259]

Glad to hear that

[@theboogiegamer3872]

Sir to be honest I did the cardboard thing with my fingers in my rooms bulb, kind of worked, pretty satisfied with the results. Physics is magic!

[@theboogiegamer3872]

Hello sir I never really understand Young's double slit experiment but after watching your practical examples, I m able to imagine it vividly. I hope every school gets a professor like you. Until then, let's watch your videos!!!! Lots of love

[@lecturesbywalterlewin.they9259]

Excellent

[@obayev]

It was thrilling to know so many important things about diffraction and its huge role in the ability to see. Thank you Professor!

[@lecturesbywalterlewin.they9259]

You are very welcome

[@plutus6316]

You are a blessing to all of us! Thank you for explaining these topics so good!

[@lecturesbywalterlewin.they9259]

You are so welcome

[@rohitgarg4065]

Sir, just curious to know ,how are you able to draw those dotted lines? :)

[@lecturesbywalterlewin.they9259]

kzbin.info/www/bejne/e36almyYbbGNeJo kzbin.info/www/bejne/fZnEqKqOmLami9U

[@noproblem4260]

its a common skill of physics teachers, just point the chalk in an angle forward and push it with loose fingers. I had a teacher who invented a special chalk kind of wet ) that after writting all over the blackboard said, " have you all copied?? at the answer of yes he ´d knock the board with his fist and the writings ´d fall magically down, great physics teacher Mr Doub at UNS Argentina

[@chastirabia1235]

Thank you professor. Love from KASHMIR🌸

[@perfectlyimperfect6849]

These lectures are true gem 💎💎 for students who want to see physics in natural world . Thankyou so much professor for bringing this beautiful lecture with full practical and explain... Loved it!! 🙏🙏😀

[@lecturesbywalterlewin.they9259]

It's my pleasure

[@Originalimoc]

27:52 Sadly I can't do the same reasoning for Theta 2. In that case why can't it be bright because start from middle of the slit then go downward there's always a Huygen source in difference of Lamda with a point in upper half. Okay I figured it out while typing this. This have to be solved recursively. Upper half + down half(for n is even but same reasoning for odd n) results in similar situation of Theta n/2. There're still peak and valley in EM field need to be added together. So the end result is dark too.

[@Daniel-OConnell]

When you have some free time , can you please explain the physics behind the 6 star like peaks around the spot of the green laser at 33:00. Am I correct that it is the same effect that makes a star have a similar pattern. I just love your physics! Thank you.

[@physicslover369]

Can we compare the spectrum of hydrogen atom described by Bohr and the spectrum form by grating which was exposed to white light...?

[@soonpaomeng]

thank you sir 😃

[@InventTwig]

professor what are the students using at 36:08

[@lecturesbywalterlewin.they9259]

a diffraction grating

[@oshikkhittt]

Thanks a bunch Mr!

[@sovonkumarnaskar7199]

Great help 😊❤❤❤❤❤

[@sgifford1000]

I just ordered an optical bench. I’ll be checking to see if you are correct. I suspect you’ll get an excellent grade.

[@BroadeningHorizonsos]

at 33:29 why the light has diffracted horizontally??? why didn't it appears as concentric circles

[@lecturesbywalterlewin.they9259]

Heisenberg makes the light ONLY spread in the direction of the small width of the slit.

[@parthkatke6706]

It's very surprising and beautiful that the same experiment of single slit diffraction can be looked at with two different aspects, mainly, Heisenberg's uncertainty principle(8.01x) and interference phenomenon(8.02x)

[@schmetterling4477]

Not really. Heisenberg's uncertainty principle does not give you the correct formula for diffraction.

[@Daredevil662]

Professor when i shined violet light on white A4 sheet it gave violet colour but when I shined violet light on your book pages, "FOR THE LOVE OF PHYSICS" it gave pink colour. Does it depend on the chemical nature of the paper.

[@flirkami]

What you are seeing is most likely related to fluorescence. The violet light is light with a wavelength of ~ 400 nm and it has enough energy to excite the chemical used on the pages which then partly re-emits light with a longer wavelength. Those colours get mixed up and your eye interprets them as the colour you're seeing.

[@Imagon100]

21:22 very rude, professor... I was born this way

[@physl2787]

If you videos on QM . whole world people will be very very thankful for you. You can change the life of people if you made video on basic Quantum physics

[@lecturesbywalterlewin.they9259]

kzbin.info/www/bejne/oouWk4OLgNRosMU kzbin.info/www/bejne/h3qUZIZmadxrmq8

[@krypton1142]

its 3am on a sunday night. i love you so much

[@aayushd824]

How fascinating for us viewers and how nostalgic for you it can be that Hubble just completed its 32 years and I got to see two fine images captured by it... One shown in the video and other published by NASA 'hickson40'

[@lecturesbywalterlewin.they9259]

www.google.com/search?q=Hubble+Iages&rlz=1C5CHFA_enUS929US929&oq=Hubble+Iages&aqs=chrome..69i57j0i13l9.10515j0j15&sourceid=chrome&ie=UTF-8

[@manassrivastava7448]

Sir today observed something very different. I don't know Sir what it is? I made a laser beam very very close to a magnet sir I observed light was showing patterns of magnetic field of that magnet

[@atharvas4399]

professor, will single slit also cause pattern?

[@lecturesbywalterlewin.they9259]

yes watch my lectures

[@sanjanasolanki9612]

Sir ,lots of love from india

[@zeyanirfan732]

Professor please tell me the relationship between diffraction and heisenberg uncertainty principle

[@lecturesbywalterlewin.they9259]

Heisenberg's Uncertainty Principle ALWAYS applies. But that does not mean that you can derive spectra formed by gratingswith only HUP. Watch my lecture where I cover HUP and watch my single slit diffraction which is consistent with HUP.

[@Vishal-vp7pj]

This video is all about dots....dotted lines, dotted spectrum, dotted grating etc.

[@harshtalajia3476]

But professor, in one of the other lectures you said that diffraction pattern is caused because of heisenberg's uncertainty principle. Are these both the same thing? It's confusing.

[@lecturesbywalterlewin.they9259]

Everything in Nature HAS TO BE consistent with HUP. The BEST/ONLY way to demonstrate that at the 8.02 level is to do the single slit diffraction. You can then PROVE quantitatively that the central image on the screen (which you can measure) is what you should expect from HUP. Other demos, like double slit interference and diffraction gratings are also consistent with HUP but that is by no means obvious at the 8.02 level. I am immensely pleased that I found on my own a way (single slit) to demonstrate HUP. Also watch this lecture with fabulous quantitative confirmation. kzbin.info/www/bejne/g5auYXeMaJdso7c

[@harshtalajia3476]

Lectures by Walter Lewin. They will make you ♥ Physics. Thanks for explaining sir. And thank you very much for sharing all the lectures and knowledge.!

[@Raudcu]

Professor, let me ask you: When use the small gratings, the retine is the screen? And i can't quite see why for seeing the maxima i can rotate my head...is the relative angle between de gratins and the source what i'm changing now? Thanks!

[@lecturesbywalterlewin.they9259]

Tell me more precisely what you and what you saw.

[@Raudcu]

Sorry. I mean, when one uses the multiple gratings sheet, and see directly to the white light with it, you say that the eye acts like a convergent lens, so i was wondering if the screen was the retine of my eye.

[@lecturesbywalterlewin.they9259]

yes, of course

[@Raudcu]

Great. And, as I can't see all the spectra at once obviously, to be able to see each maxima i have to turn my head. In this case i'm changing the relative angle between the source and the gratings instead the angle between the gratings and the screen?

[@lecturesbywalterlewin.they9259]

yes

[@joaquinzurschmitten6041]

Sir, the human angular resolution that you mentioned is for only one eye? Or is it calculated for both eyes? (When you have 2 telescopes it is equivalent to have only one bigger telescope, with the eyes happens the same?)

[@lecturesbywalterlewin.they9259]

with 2 telescopes you get twice as many photons, but if yu can use them as an interferometer you get twice as high a resolution. Your eyes cannot be used as an interferometer thius 2 eyes doess not increase your resolution

[@subhamoychakraborty7367]

Why does a vertical slit produces a horizontal diffraction pattern in the screen (i.e intensity variation along horizontal direction which is perpendicular to the slit's orientation) and vice versa?

[@lecturesbywalterlewin.they9259]

Heisenb's UP

[@adityaanand1920]

What happens on critical angle, such that a transparent medium starts to act as reflective surface? Please consider it

[@schmetterling4477]

You are losing that light. Where do you see that transparent medium, though?

[@manassrivastava7448]

Sir we know that electric field start from +ve charge and ends at -ve charge. Change in electric field produces magnetic field. Magnetic field start's from North pole and end at South pole and form closed loops. Change in magnetic and electric field due to acceleration produces Electromagnetic radiation. Sir but why light travels in straight line and do not form any loop.

[@lecturesbywalterlewin.they9259]

I suggest you ask quora. I am sure they have discussed this at length. Or try google.

[@randomvideosbyarandomguy2709]

Are there any lectures on fresnel diffraction and cornu's spiral? Great pic on channnel logo btw👍

[@lecturesbywalterlewin.they9259]

search on line: OCW, edX, Udacity, Coursera

[@manassrivastava7448]

Sir if make a charged metallic ball freely fall in vaccum with it produce electromagnetic radiation

[@lecturesbywalterlewin.they9259]

That is an excellent question. It has been debated for decades. The central strangeness is that a free falling charge emits no radiation as seen by a free falling observer, while a supported charge emits no radiation as seen by a supported observer. On the other hand if you are standing on the earth then you will see a free falling charge as radiating. And if you are falling freely then you will see a charge that is sitting on a table as radiating. This means that radiation exists for some observers and not for others! That seems very surprising, but is in fact the basic result that leads to Unruh radiation (or Hawking radiation if you think gravity).

[@manassrivastava7448]

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

[@sushantaadhikari6692]

I asked the same kind of question three years ago to our professor. He didn't give the answer.I thought harder.And came to the conclusion that two different result will indeed be observed.

[@BartdeHaas]

Professor Lewin, ik vroeg me af of het fenomeen dat we waarnemen na 17:30 minuten in het college (het inferentiepatroon van wit licht, als ik het goed begrijp.) iets te maken heeft met de eerste (primaire) regenboog, omdat daar de kleurenvolgorde ook van blauw aan de binnenkant naar rood aan de buitenkant loopt. Zo ja, is de verklaring daarvoor te vinden met Google (dan zal ik daar gebruik van maken)? Zo nee, mijn excuses.

[@lecturesbywalterlewin.they9259]

17:30 diffraction grating, it shows dispersion! angle is proportional to (lamda/d) thus the larger lambda the larger the angle. Thus blue is on the "inside" red on the outside. Rainbows show dispersion due to the geometry of the water drop (NOT lambda/d, but lambda) for the primary bow. Thus red is on the outside and blue on the inside. For the secondary bow dispersion is reversed due to geometry, thus blue is on the outside and red on the inside.

[@pascalmathieu9332]

True physics without lies.

[@schmetterling4477]

??? What is it with you and lies?

[@physl2787]

Sir why the phase difference has to be constant between the two wave to see the interference pattern

[@dishakhurana6739]

Because if the phase is changing At the same location you will observe constructive intereference as well as destructive interference. Due to which you will not be able to observe intereference pattern properly!

[@InventTwig]

professor what all was included in the optics kit of 8.02 and 8.01

[@lecturesbywalterlewin.they9259]

4 linear polarizers 1 circular polarizer 1 quarter wave plate 1 half wave plate 6? colored filters

[@InventTwig]

Thank you prof :)

[@nehakhanra9853]

Hello sir, with profound respect. Sir, what is meant by the geometrical shadow and why do we introduce it into the definition of diffraction?

[@lecturesbywalterlewin.they9259]

I do not know that "geometrical shadow" is. try google

[@chiragvashist8415]

33:29 Difraction Baba goes crazy....... #MomentOfTruth

[@amritha.p.b9747]

Feels like... IIt professor

[@joaquinzurschmitten6041]

Hi! How can I produce a "homemade" diffraction grating? I loved your lecture!

[@lecturesbywalterlewin.they9259]

Not possible - but a CD can be u,sed as a reflection grating

[@schmetterling4477]

How much effort do you want to put into it? Gratings can be made mechanically or photographically. Making them mechanically is easier. You can make a small machine that can scratch gratings into a silver mirror from simple mechanical parts that are now available cheaply on the internet. Having said that, cheap industrial gratings cost less than $5 and even a well made optical grating that would be useful for many actual science experiments is less than $100 these days. You can buy ready to go pocket spectroscopes for $20...

[@flyingbirds6794]

Do Sir, does the single slit really behaves like there are a lot of light sources when light passes through it?

[@lecturesbywalterlewin.they9259]

diffraction!!!

[@flyingbirds6794]

@@lecturesbywalterlewin.they9259 so it does doesn't it?

[@sb-hf7tw]

Professor, 1- Is there any video about measuring polrazation angle with the help of Brewster's law by a prism spectrometer ? And 2- video about concept of Schuster's method parallel ray setting ?

[@lecturesbywalterlewin.they9259]

1. yes in my 8.03 lectures 2. no - use google

[@sb-hf7tw]

Sir, which video in your 8.03 lectures, lecture number please?

[@lecturesbywalterlewin.they9259]

all my lectures have listed topics. go for it

[@sb-hf7tw]

Thank you, sir

[@atharvas4399]

Dear professor, 1 can only electric or magnetic waves exist independently? 2 also for EM waves, do the E an B waves need to be in phase? that is would an EM ave travel if E and B were out of phase or had a different frequency? 3 Does the fact that light travels slowly in mediums with high n, conflict with relativity? I found an explanation online of why not, but i am not sure of its authenticity. It said that light has to take a longer path in mediums with a high index of refraction. Is this correct professor? Thank you very much.

[@lecturesbywalterlewin.they9259]

use google

[@slyhawk57]

Professor, when looking through a grating at a white light source why does the zero order have color to it? The color also shifts with the angle of the grating in relation to the zero order usually from a beige/yellow color to purple/blue. Thanks!

[@lecturesbywalterlewin.they9259]

The zero order of a light source is has the same color as the light source. If not, tell me how many minutes into the lecture this was not the case.

[@slyhawk57]

Oh I'm sorry this is from my own observations, I manufacture diffraction gratings for a living and when you hold them up to your eye the zero order has a yellow color and will change dependent on the angle you look through it. We have been unable to work out why to this point. I am just uploading a short video of the effect. Thanks!

[@lecturesbywalterlewin.they9259]

watch my grating lectures! I use white light, the zero order is white. Your gratings may be of low quality and the plastic may absorb part of the spectrum changing white into yellow.

[@slyhawk57]

I have been they are wonderful thank you so much. They are a 100l/mm grating blazed for 500nm. I think you must be right and it must be the resin we use for the replicating process. Without the grooves however it appears clear. Thank you so much for taking your time to reply.

[@slyhawk57]

Having now compared the blank film and the same film with a grating on it seems the issue is not the resin or the grating quality as these were some of the best the NPL ever produced. The grooves seem to be creating a color filtering of the zero order. Hopefully after watching more of you lectures I will be able to understand why this is happening.

[@hindusthaniboy]

If it's means than interference is a tool of defracction? Love from India 🇮🇳. Your virtual student saptarshi pahari.

[@anmolraina3509]

Urgent question..... we are taught in school that when two light rays intersect or appear to intersect, a real/virtual image is formed. 1. if a real image is formed, then what exactly happens at the junction of the rays? If it is true then why don't ordinary intersection of light rays form images? 2. If a virtual image is formed, then the confusion build up even more. We usually extend the rays of light which reflect and don't meet, but the extended rays seem to meet behind the mirror and we say that a virtual image is formed. Here the rays don't even meet, then how the hell is an image formed !!!!! ...... Please reply me sir/intellectual KZbinrs, as I'm always sent out of the class for asking these questions in the class, please help me asap.

[@lecturesbywalterlewin.they9259]

questions unclear use google about the meaning of a real image and a virtual image,

[@swanhtet1]

Sir, Why the difference in path length of the two Huygens sources at 27:00 is lambda?

[@lecturesbywalterlewin.they9259]

constructive interference

[@swanhtet1]

Sir, i thought constructive interference is the result(or the consequence) of the difference path length, lambda. So please may i know what is the initial reason that the parh length difference of the two sources (at those points) are lambda?

[@lecturesbywalterlewin.they9259]

interference is the result of different path length. If the difference is lambda/2 of 3*lambda/2 or 5*lambda/2 . . . there is destructive interference. If the difference is n*lambda n is an integer (including zero) there is constructive interference. It's well explained in my lectures. If you need more help, please use google.

[@swanhtet1]

Sir, I understand that path length difference lambda will cause constructive interference. You did explain it well and we appreciate you for that. However, I am not asking why the constructive interference was formed. What I am really really asking is why those two points have path length difference lambda? Please kindly click this link (imgur.com/a/eMaYtzT ) to see the two points that I am referring to. I did tried to find the answer for that in google but nobody else can teach this better than you, sir.

[@swanhtet1]

Thank you, Sir.

[@qinyuping2823]

Sir, I can see things right in front of me more clearly than things by my side. So does this mean human eyes have different angular resolution in different direction, or it's just a matter of neurology?

[@qinyuping2823]

I am thinking of an explain after posting this question. Given the angle between your line of sight and the line from your eyes to the object you observe θ, and the actual area of your pupil S. Then the effective area of your pupil will be S sinθ, so the angular resolution will drop. Will this explain work?

[@lecturesbywalterlewin.they9259]

watch my 8.03 lecture where I discuss angular resolution of eyes of my students in lecture *with demo*

[@qinyuping2823]

@@lecturesbywalterlewin.they9259 Thank you sir

[@physl2787]

Sir why when the two opening in double slit have not constant phase difference than we not see interference pattern

[@lecturesbywalterlewin.they9259]

question unclear -

[@daniyalali3428]

Professor, he means to say " why non-coherent sources of light (passing through two slits) produce an interference pattern that is difficult to observe" :-) ......

[@lecturesbywalterlewin.they9259]

yes 2 sources of light passing through 2 narrow slits will produce interference patterns. KEEP in mind that one photon every sec will produce interference patterns if you record the location on a screen with CDs and you wait a week, you will see a super interference pattern.

[@physl2787]

Lectures by Walter Lewin. They will make you ♥ Physics. What will happen after a week

[@lecturesbywalterlewin.they9259]

you will then have accumulated about half a million photon and the interference pattern øn your CD's will be very cool!

[@yash29210]

I saw a video in which they used a sensitive device which measured the number of photons falling on it.........so they used it to perform "SINGLE PHOTON INTERFERENCE" through a double slit....... It was assumed that since there is only one photon so there will be no sign of interference patterns because there is only one photon.............. After 1 sec of the beginning of the interference, the graph of the number of photons counted striking the instrument vs position across the canvas (in mm) was very RANDOM.....................showing that there are no interference patterns........... But when we added up the results over a period of time then we saw that the pattern observed was same as we used to see in case we passed many photons through the same double-slit............ AND IN THE END, IT WAS CONCLUDED THAT PHOTON IS NEITHER A PARTICLE NOR A WAVE................... . . . . . . . . . WHAT ARE YOUR THOUGHTS??

[@lecturesbywalterlewin.they9259]

Single photons (1 per sec) will give an interference pattern in 2 slit demo. That's puzzling for all who think Newtonian (as we all do). But it's the way the world ticks. To ask whether photons are particles or waves is a meaningless question as it is a Newtonian question. use google for more insight.and watch this lecture by Feynman kzbin.info/www/bejne/aJ6snGatd693nac

[@jonathanhughes380]

Professor I would argue the fact with you if the intensity of the light source is too high the closer together it is the eye cannot differentiate between 2 points but further away the I can because intensity decreases also you intentionally chose blue LEDs human eyes are very susceptible to Greens and Blue wavelengths had that demonstration been done with more wavelengths of colors through the whole Spectrum I believe the results would have been very different and had you change the intensity you would have seen different results and got different hands waved of what people could see

[@physl2787]

Sir please help the whole world by making course on Quantum physics.

[@lecturesbywalterlewin.they9259]

kzbin.info/www/bejne/oouWk4OLgNRosMU kzbin.info/www/bejne/h3qUZIZmadxrmq8

[@surendrakverma555]

Very good lecture 🙏🙏🙏🙏🙏🙏

[@User-jr7vf]

when were these lectures performed?

[@lecturesbywalterlewin.they9259]

8.01x 1999, 802x 2002, 803 2004

[@User-jr7vf]

thank you

[@ewanofscotland]

7:07 "3 times more sources, 9 times more light" 7:26 "3 times more sources, 3 times more light" Think there was a slip up there....

[@manuferre7186]

I am a great follower who has always enjoyed his classes, and to the point that he has already sent me a signed photo for my birthday, a few months ago, to Spain. Today I bring you a PROPOSAL, not a question: Would you be willing to visit us (by video call) at Quantum Society, the largest university science and technology association in Spanish language? We would love to chat with you, ask you questions about physics, knowledge and education (how to improve it, what do you think needs to be added in today's classrooms). Also, everything would be posted on our KZbin channel to be accessible to the whole world. In case there are any doubts, the interview would be in English. As a side note, I am a physics student at King's College London, right where MAXWELL published his equations in 1865 that would revolutionize the world, and so it would be an honor to talk to the person who taught me to understand his theory better than anyone else (and lose my virginity! hehehe). Thank you

[@lecturesbywalterlewin.they9259]

I do Live video Q&A with students in classes (with about 25 - 50 students) all over the world. This requires a lot of preparation but I have done it with Sweden, Turkey, The Netherlands, Russia, Canada, US. If one of your Physics Teachers approaches me here in my KZbin channel we can try to work something out.

[@lecturesbywalterlewin.they9259]

I do Live video Q&A with students in classes (with about 25 - 50 students) all over the world. This requires a lot of preparation but I have done it with Sweden, Turkey, The Netherlands, Russia, Canada, US. If one of your Physics Teachers approaches me here in my KZbin channel we can try to work something out.

[@manuferre7186]

@@lecturesbywalterlewin.they9259 I will try to do it sir :)

[@Originalimoc]

What's wrong with Dutch 😂

[@taimoorahmad4540]

U wrote u will statt to love physics but i’m starting to hate physics more what a disstressing subject.

[@lecturesbywalterlewin.they9259]

stop watching my lectures!

[@schmetterling4477]

Not everybody needs to be a physicist. We also need armchair NFL coaches.