8.02x - Lect 30 - Polarizers, Malus' Law, Light Scattering, Blue Skies, Red Sunsets

[@jbryan09100]

Your sunset demonstration was simply amazing to behold. If only more professors adopted practical demonstrations like these to tie in the concepts behind them. Thank you for posting these lectures!

[@lecturesbywalterlewin.they9259]

:)

[@zacharyharter8060]

I was just a newborn whenever you did these lectures. Now I'm an EE student. I wish I could shake your hand for putting on such a jolly good show!

[@lecturesbywalterlewin.they9259]

Wow, thank you!

[@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!

[@ramoncrouch]

Lectures by Walter Lewin. They will make you ♥ Physics

[@jerrylu3158]

After so many years I finally understand why sky is blue:)

[@cayezara8110]

This blew my mind. What a practical demonstration! It is fascinating and mind blowing!!! What a brilliant Teacher your are, Professor Walter Lewin.

[@profamitgupta]

I have never been fully able to comprehend polarization of light using polarizers. Prof. Lewin your lectures have really got me motivated to figure this out now based on the theory presented in this lecture.

[@darrylwillard2419]

I was really surprised when you lighted up the cigarettes. You can't do that now. Sacrifice for science sake. You are a great lecturer Professor Lewin. 😁😁😁

[@lecturesbywalterlewin.they9259]

:)

[@scienceandphilo]

Professor Walter Lewin I had a dream last night. I was on a bike with Richard P. Feynman. I was so happy that I was with a famous scientist. Richard P. Feynman also told me that he was not only a scientist but he also composed some 150 symphonies. I said to him I listened to one last night and it was great. He said it was his symphonies. I dont know how to interpret this dream.

[@lecturesbywalterlewin.they9259]

super dream - He was a famous drummer

[@cameronbrown7231]

I’m almost done with my undergrad in aerospace engineering, but I would still love to go back for a physics degree at some point. I always return to these lectures and am just so fascinated.

[@JensenPlaysMC]

Absolutely AMAZING demonstration!

[@priyanshuuu282]

i feel like this is the first time i have understood physics and can finally relate with people who say that they love physics because now i clearly can see and feel the reason. Thank you very much sir😊

[@kunalmodi6814]

Thank you sir for your contribution... Your lectures helped me a lot in improving my grades...

[@PymGordonArthur]

I literarly feel love for this man.

[@AlirezaNabavian-eu6fz]

I wonder why they haven't given you a noble prize yet...millions of university students will vote for you if there was a democracy....but you have won our hearts and minds...and this is coming from an EE engineer

[@darushhaghnazar1543]

I really Loooove Your Lectures. They Solved lots of my questions about electromagnetic phenomena. Wish you the Best

[@lecturesbywalterlewin.they9259]

Happy to hear that!

[@obayev]

What an amazing sunset it was indeed! Thank you Professor!

[@lecturesbywalterlewin.they9259]

Yes it was!

[@not_mrfrost]

Wow, these lectures were made when, in 2002? I was just a newborn then, and now, I'm watching these lectures for the Physics exam, in an Italian university!

[@lecturesbywalterlewin.they9259]

keep it up!

[@perfectlyimperfect6849]

Thankyou so much sir for proving that physics is really study of nature What a wonderful lecture. LOVED IT!!

[@lecturesbywalterlewin.they9259]

It's my pleasure

[@ElectromecanicaIndustrial]

The Greatest Of All Time

[@sanskarkumar8674]

He is great. Love you so much Sir

[@tunatercan]

Professor, when my teacher taught me the interaction between photon and electron, he said that it must be in the rules of Compton scattering. For calculating the properties of scattered photon and electron he solved two equations: conservation of energy and conservation of momentum. But I always thought that it must be scattered like your way. The point i am confused now is: Which one is correct and what is the difference between two methods? ❤😊

[@lecturesbywalterlewin.they9259]

yes, that is Compton scattering - I suggest you google it

[@darrylwillard2419]

Professor Lewin, I was watching a lecture by Leo Susskind on You Tube. It was his first lecture on quantum physics. I just wanted to let you know that he made a statement that there was another guy who had his physics lectures on the internet, I said that the guy was really good. I think he was talking about you. He recommend that viewers watch those lectures before watching his lectures on quantum physics. He unfortunately didn't say your name. I figured that it must have been you because you are the only other person that have lectures on KZbin.

[@lecturesbywalterlewin.they9259]

:)

[@dogscats8141]

I really enjoyed your experience on dispersing cigarette smoke. I also see smoke in blue. I have a question for you, professor. Why do people say that some things, for example, trousers, are black, while I see them as dark blue?They say that women's eyes can distinguish up to 2 million shades of colors, maybe for this reason, I see many more colors than others.

[@destinationastudypoint2195]

Became overwhelmed sir hatsoff you sir

[@surendrakverma555]

Excellent Physics professor. Regards

[@lecturesbywalterlewin.they9259]

Many thanks!

[@sandypatel2511]

Wow amazing lecture

[@kalpanaghartimagar2301]

Smoking is injurious to health !!! Hats off to you sir 👏👏

[@sandeeppatidar1106]

Thank you so much professor

[@fjs1111]

Really interesting as always. Learned quite a lot.

[@lecturesbywalterlewin.they9259]

Glad to hear it!

[@onemoreday2go]

The camera-man who gets free MIT education...

[@ashutoshbhakuni303]

Thank you for the great lecture sir! A few big confusions in my head: 1. Why dont the water molecules in the container scatter light? Shouldnt they behave like gas molecules in atmosphere (with regards to their size)? By the same account why does ocean look blue but a glass of water doesnt? Or why air around us on ground where atmosphere is more dense not look bluish? Does quantity of medium has something to do with it? If yes then why does the thiosulphate solution scatter in the same quantity of solution? Is it because Tyndall effect is much more stronger than Rayleigh scattering? Similarly in cigarette demo why did light coming from box look white- shouldnt air molecules scatter light like cigarette particles like it happens in atmosphere? 2. What is difference between Tyndall and Rayleigh? Sky is blue due to which of the two phenomenon or both? 3. Do smoke particles from cigarette and from incense stick differ in size as I find the latter's smoke white?

[@lecturesbywalterlewin.they9259]

1 and 2 use google 3. question unclear

[@ashutoshbhakuni303]

3. When I see the smoke from an incense stick it seems white unlike the bluish smoke in the video. Is it because particulates from cigarette and from an incense stick are of different sizes? Also when I blow at the burning tip of the incense stick I see the bluish smoke. How is that?

[@lecturesbywalterlewin.they9259]

yes, must be

[@KeithandBridget]

For anyone who is unsure about Walter Lewin's problem 188 the first ten minutes of this lecture is where to look for insight.

[@umerhayat1590]

Respect for sir wlater lewin from pakistan

[@apexinstituteforphysicscon8979]

Good morning professor, I have one confusion that is ,when light enter into water from glass why electric field amplitude not change while energy density decreases, is electric field not depend on relative permitivity

[@yogeshkumar-zv1ix]

sir, the demonstration that you did for making the reflected light polarised when the incident light travelled from air to glass(brewster angle concept), so does it mean here that for the very special case of theta1 + theta 2 = 90 degree, that glass is behaving like a polariser and also does the brewster angle concept works only for the above-mentioned case or for any of the case?

[@lecturesbywalterlewin.they9259]

tan(theta brewster) = n2/n1 watch lect # 18 of 8.03

[@yogeshkumar-zv1ix]

@@lecturesbywalterlewin.they9259 ok sir

[@arwadewaswala3747]

hello professor i have apple ipad when i saw its bright white screen through my polarised sunglasses i saw partial polarisation but even i could able to see colorful spectrum is this related to polarisation ? and why i saw colorful spectrum on my screen sorry if the question is trivial

[@lecturesbywalterlewin.they9259]

+Arwa Dewaswala It's not a trivial question. LEDs are polarized and the colors are probably the result of birefringence. I suggest you search the web for a proper explanation.

[@jonathanhughes380]

Professor Lewin, looking straight up into the night sky at stars and planets, Could you also conclude. That stars and planets that appear more Red are farthest from earth, and ones that are closest to the earth appear more blue?

[@lecturesbywalterlewin.they9259]

use google that's what it is for.

[@jonathanhughes380]

@@lecturesbywalterlewin.they9259 you can always trust what you read on the internet it's like saying if it's on the internet it must be true.

[@somyaarora7402]

kudos to the cameraman for showing us the polarisers crossed

[@sakshiraut5406]

Thank you Sir for improving me.

[@GaganpreetSingh-ft1xi]

Sir in which lecture has you demonstrated linear polarization of EM waves?

[@lecturesbywalterlewin.they9259]

it's in one of my 8.03 lectures

[@astitvagarg8997]

Hello sir I could not get how did you decompose the coming light in two directions isn't the coming light or the incident beam already in the plane of board?? And also why should the two components broken be equal for unpolarized light?? I am not getting it pls explain sir.

[@lecturesbywalterlewin.they9259]

watch my 8.03 lecture in which I cover refection & refraction + polarization in great detail. I even derive 1 of the 4 Fresnel's equations.

[@abdullaalmosalami]

He decomposed the electric field; the arrow used to represent the "light" is merely the direction of propagation! The E field and B field that make up the light are of course perpendicular to the direction of propagation.

[@darinpeev5820]

Superb!

[@Raudcu]

Professor, what kind of light did you use to the sunset demo?

[@lecturesbywalterlewin.they9259]

Carbon arc

[@Raudcu]

Thank you! And the thiosulfate is thiosulfate sodium?

[@lecturesbywalterlewin.they9259]

yes, sodium thiosulfate

[@kateoversby8177]

Quick question: if two polarized lights with the same polarization and wavelength meet each other at 90 degrees (because their sources are at 90 degrees), what happens? do they cancel each other out or just normal interference?

[@lecturesbywalterlewin.they9259]

+Kate Oversby be more specific about the set up. If the 2 light sources are independent, thus NOT coherent, then they cannot interfere in any way.

[@kateoversby8177]

+Lectures by Walter Lewin. They will make you ♥ Physics. Thanks. What I mean is: a polarized blue light is released from a source, it hits a particle and gets scattered, some of it at 90 degrees. What is the interaction between more photons coming from the original source and this one, now tilted 90 degrees? thanks

[@lecturesbywalterlewin.they9259]

+Kate Oversby Question unclear.

[@kateoversby8177]

Hmmm, let's see: a vertically polarized light travels from a blue LED. Once it's in the air it hits particles and gets scattered in all sorts of directions, including 90 degrees. I'm assuming it will remain polarized in all directions, and especially at 90 degrees, where it would be anyway right? So what happens when more photons, released from the same light source, hit this perpendicular ones, that have been bounced at 90 degrees from the particle? Will there be destructive interference? Or a mix? Thanks for replying and please let me know if any specific point is still unclear.

[@lecturesbywalterlewin.they9259]

+Kate Oversby First of all this experiment cannot be done. It may be possible to do this very differently with tens of thousands of photons but that is not even clear at all. In the highly unlikely event that a similar experiment could be done, there will be no interference between the thousands of "primary" photons who were polarized and who all scattered at 90 degrees and the thousands of secondary photons who then "interact" with the prima photons. There will be NO interference as there is no coherence in photons from LEDs. I stress, however, that this experiment cannot be done.

[@InventTwig]

Professor , what was included in the optics kit ?

[@lecturesbywalterlewin.they9259]

2 linear polarizers, a diffraction grating, a quarter wave plate, a circular polarizer and colored filters.

[@InventTwig]

thank you professor :)

[@anandapatmanabhansu]

Sir can the clouds with watermolecules also can be red

[@kylebrinker9422]

Mr. Lewin, I can see where you got the 1/2 in Maulus' Law (with the HN 50), but in physics problems where the HN isnt listed do we just assume its 100 then?

[@abdullaalmosalami]

HN 50 is already the maximum man...

[@poojyadav7442]

while in a thunderstorm, the cloud appears to black because it absorbs all the color which are coming from the sun? @walter lewin

[@Anand70707]

Professor, if light gets polarised due to scattering by dust particles, does that mean that all light incident on our eyes is more or less partially polarised?

[@lecturesbywalterlewin.they9259]

Castor Gemini You are right! All light that has been scattered by particles much smaller than 0.1 microns is linearly polarized to some degree. If the scattering angle is 90 degrees it's 100% linearly polarized.. If the angle is 0 or 180 degrees the percentage of linear polarization is zero. .

[@jasinsworld3095]

I was looking for converging lens, did u cover about this topic

[@marufhasan9831]

Sir I have a question. Why doesn't the polarizer create its own radiation? The electrons inside the polarizer should oscillate when the sun's rays come in and so we should get light or electromagnetic waves of the same frequency from the polarizer just as it did in the scattering of light.

[@lecturesbywalterlewin.they9259]

use google

[@marufhasan9831]

Sir I used Google but got no satisfactory answer there.@@lecturesbywalterlewin.they9259

[@neojohns2487]

Sir I have a question...i have a light source, I have placed two polarizer(p1 and p2) before that light source...and p2's axis is 90 degree with p1(cross polarizer)...now all light should be stopped..now if I put a solenoid between p1 and p2 , where solenoid has been placed on the direction of the light..now if I switch it on, will it affect the light any way, will p2 allow any light

[@lecturesbywalterlewin.they9259]

work out the physics and send me your result.

[@cpuiaralte3156]

awsome vedio, but subtitle could not be displayed on downloaded vedio????

[@jpskiskate2000]

what a nice sunset

[@anandapatmanabhansu]

Sir in 43:06 who can see the blue light scattered from sun

[@ahmadeldesokey9844]

Sir , why the moonlight is white and not red ? My explanation is that light coming from the moon has to travel through many atmosphere and thus should not it be red ?

[@lecturesbywalterlewin.they9259]

when the Moon is close to the horizon it too is red and so are all planets and most stars

[@InventTwig]

does anyone know why only electric fields are taken into account while doing polarization. is there no magnetic fields ?

[@lecturesbywalterlewin.they9259]

+InventTwig The magnetic field polarizes orthogonal to the electric field in free space. We generally only talk about the electric field because Maxwell's equations define a one to one relationship between the two. It would make just as much sense to only talk about the magnetic field. We choose the electric field because, in general, when light interacts with matter it is the electric field which causes all of the interesting effects.

[@InventTwig]

+Lectures by Walter Lewin. They will make you ♥ Physics. Thank you Professor Lewin :))

[@astitvagarg8997]

The explanation you gave sir for the light being scattered in a plane perpendicular contains 100 percentage of linear Polarized light.. the point you are trying to make is all photons that will have there E vector in that perpendicular plane will cause electrons to accelerate in the direction of E vector in that plane and because of that acceleration of electrons a new photon now will be re radiated at at point on the circle having a fixed direction of E vector ?? Am I correct professor in my understanding?

[@lecturesbywalterlewin.they9259]

question unclear. Watch my 8.03 lecture in which I diucuss reflection and refraction (including polarication) in great detail and I even derive one of the 4 Fresnel eqs. kzbin.info/www/bejne/lXWUq2mqZ99kbJo

[@astitvagarg8997]

@@lecturesbywalterlewin.they9259 sir u took a unique plane and all coming photons in that plane cause electrons present in dust particles to accelerate along their E vector , this accelerated electrons will re radiate a photon with a fixed direction of electric field am I correct ??

[@lecturesbywalterlewin.they9259]

question unclear - how many minutes into the lecture?

[@astitvagarg8997]

@@lecturesbywalterlewin.they9259 31 minutes sir

[@lecturesbywalterlewin.they9259]

>>>and because of that acceleration of electrons a new photon now will be re radiated>>>> the acc of an electron causes EM radiation, not just one photon.

[@canned_heat1444]

Ok I have a very silly question: if a laser that is emitting a laser beam moves at a certain speed, relative to him would the em wave oscillate with respect to time or would it still be fixed? My intuition is that it must be oscillating, cause if fixed the beam would be moving at a speed greater than the speed of light. (This experiment is done in a vacuum). Sorry for the silly question but it has been bothering me. Thanks

[@lecturesbywalterlewin.they9259]

laser light, like all light, will increase in frequency when it moves towards you or when you move towards the light and decrease when you or the light recedes. It's call relativistic doppler shift.We talk about blue shit ir red shift I cover this in my lectures - or ise google

[@canned_heat1444]

Ahh yes, of course! I remember now your 8.01 lecture on doppler shifts. Thanks a bunch!

[@cottoneye2012]

Professor, Why is orientation of polarized light same as orientation of electric filed vector?

[@lecturesbywalterlewin.they9259]

that's by definition

[@rihaveinaiba8269]

Sir i can not explain it to myself why when i was kid i saw a huge red moon not close to the horizon but above my head. Was it due to the moon position compared to Earth?

[@wayneliang4524]

it is likely that you saw a lunar eclipse, where the earth is between the sun and the moon, as most of the light is blocked by the earth, the light that reaches moon mainly passed through the earth's atmosphere ( try to draw a graph or look it up online if you don't understand why, it is hard to show with just words.) for the same reason of sun being red during sunset mentioned in the lecture, the moon 'sees' red light and therefore reflects red light. hope this helps you

[@adityasalunkhe8156]

so if circularly polarized light from the sun contains uniquely oriented and linearly polarized electric fields and we call these photons then do each of those photons have a different wavelength and oscillate at a different frequency?

[@lecturesbywalterlewin.they9259]

if light is circularly polarized the E-field in the EM wave is also circularly polarized. That's trie for any wavelength

[@yash29210]

around 12:35....in this case what will be the direction of B-field of travelling EM wave?

[@lecturesbywalterlewin.they9259]

direction of B field is here always perpendicular to the direction of propagation and to the direction of the E-field. Such the EXB vector is in the direction of propagation.

[@tanveersingh8290]

why polorized light dont get distored by external sources of light?For instance lets take 2 points one is x(Polorized) and y(unPolorized).So will x get affected by the point y?

[@lecturesbywalterlewin.they9259]

question unclear - watch my 8.03 lectures on polarization

[@abdullaalmosalami]

Waves in general do not interfere with each other I believe and in a way just "pass through" each other. However, the physics of light is much more involved so yeah...

[@yash29210]

can we differentiate between a polarised light and a non-polarised light through naked eye?

[@lecturesbywalterlewin.they9259]

yes we can. google haidinger's brush

[@darrylwillard2419]

If you use a blue light filter is it the same as using a polilizer?

[@lecturesbywalterlewin.they9259]

NO a filter does not polarize light

[@vs-cw1wc]

But why is the sunset blue on Mars?

[@someguy9046]

Mie scattering en.m.wikipedia.org/wiki/Mie_scattering

[@jivika2638]

37.16😂 you are great Sir👍😀

[@linseito2530]

so much fun!

[@ajathreya5005]

at 11:54, Why we have to decompose the unpolarised light into components that are perpendicular and parallel to the plane of black board. What if the parallel components are not in the plane (other than parallel) ?? Do they are for our convinence.??

[@lecturesbywalterlewin.they9259]

watch my 8.03 lectures. The answers are there.

[@ajathreya5005]

OK, thank you sir.

[@ahmadeldesokey9844]

Piece of art .

[@kalpanathakur2726]

sir isnt there a lecture for ray optics

[@lecturesbywalterlewin.they9259]

What is "ray optics"? Try this lecture: kzbin.info/www/bejne/n3y4hIqAjLKGoZY

[@shivratanjat1359]

Grand

[@yash29210]

at 4:26 you tell 50% gets through....where does rest 50% goes?

[@lecturesbywalterlewin.they9259]

most absorbed some reflected

[@rohitsanjay1]

why does scattering occur at an atomic level?

[@lecturesbywalterlewin.they9259]

physics.stackexchange.com/questions/83105/explain-reflection-laws-at-the-atomic-level

[@odal6770]

kzbin.info/www/bejne/m4jFgIt3er-sh6M If we compare the first and the second demonstration, the one with the double writings, and the one with the double spots, we will discover striking similarities that could help us understand the process. 1) single bright beam is text as it would appear directly without the quartz. (not shown) 2) with the quartz, there is one bright intersection of two faint spots. For the text, the dark brown spots are where both writings intersect, while the faint letters correspond to the faint spots. 3) using a polarizer shows either one of the two spots or writings. What is the logic behind this process? It might be easier to start with the spots. How does the quartz duplicate the beam? But mostly, could we do it with other means? Let us first notice that only the part of the spot that does not intersect with the other spot disappears from view. The remaining spot has a much brighter part where both spots intersected. The intersecting part seems to be the same for both spots, something that can be expected. First, let me remark that it is not difficult to duplicate a spot. Two reflecting surfaces at an angle less than 180 degrees would easily do the job. In fact, it would work even better if there was a third reflecting band between both surfaces onto which the beam is oriented, and in such a way that a part of the beam would be reflected by each mirror. This way, we would keep a central bright portion, and two fainter parts. The trick is now to attenuate, alternately, the intensity of one of the reflections with a polarizer. Something we have already learned to do. Replace the beam with a writing, and you will get the first experiment. Don't forget that even if this analysis is correct, it still does not tell us how the quartz does it really. Only that it is possible to simulate the effects.

[@bedresamarth]

Hello Walter please clarify one thing .......I heard my teacher say that since violet has a shorter wavelength it does get scattered first and the colour of the sky is actually violet .....but the our eyes are not that much sensitive towards the violet light and since scattering takes place at a large distance from our eyes ...so the colour of the sky appears blue to us ...........this was the statement given by my teacher......Is this correct or is there something else

[@stargazer7644]

The sky color isn’t violet because there isn’t as much violet light from the sun as there is blue, and your eyes aren’t as sensitive to violet.

[@mustahsonkhan7835]

sir will you tell me what is polarisation in very simple words????

[@lecturesbywalterlewin.they9259]

please learn to use google

[@hariomsharma7674]

sir, why reflected ray is polarised and refracted ray is not? i searched it on web but didn't got any satisfactory answer.

[@lecturesbywalterlewin.they9259]

That is explained in one of my 8.03 lectures. I calculate there the Brewster Angle.

[@rontalamahender8966]

9:40 funnies part

[@lucassggabriell]

That part of the cigarettes was way too gross. Don't you know as scientist about the risks of cigarettes? I'm sure you could have done another experiment. Very disappointed...