This tests your understanding of light | Optics puzzles 1

[@SteveMould]

Can't wait for the final video! As always, your animations add so much clarity.

[@berkekus7053]

Comment under steve mould for sale

[@GooogleGoglee]

❤

[@didodido883]

They do.

[@d.mort.]

Hey Steve, Dave here. I think a wonderful follow up to this video would be about why the polarizers in this video are in the wrong direction and the reasons behind the common misconception. I am an electrical engineer with a passion for light, I would be happy to collaborate!

[@gregorywitcher5618]

*helically twisted* clarity, my favorite.

[@Epiminide]

Fun fact: Measuring the angle between the two filters can provide an estimation of the amount of sugar in the solution. This trick is applied to precisely evaluate the amount of sugar in wine grapes and must. These animations are awesome!

[@seth094978]

Is that the brix percentage?

[@Epiminide]

@@seth094978 I think Brix measurement is based on refraction properties rather than polarization. But I am far from being an expert in this field.. Let's wait if someone can clarify this point..

[@rafael_l0321]

@@seth094978 He is refering to a polarimeter, which uses a specific light frequency (to avoid the phenomenon shown in the video). As far as I know, °Brix (degrees, not %) is usually measured with a refractometer, but you can convert the concentration measured with a polarimeter to °Brix

[@ricgreen1]

Could we use this technique create a non invasive glucose monitor for diabetics?

[@amruthchangappa]

@@ricgreen1 I don't think so, you would still need a blood sample for the polarimeter, and blood has many other substances in it that would prevent this from working.

[@quinnphys]

Hey folks, Quinn here. Lots of people have been asking how they can make this demo themselves, so here’s how I built it: Materials: ⁃ Table sugar (sucrose) ⁃ Water ⁃ A glass tube that you can fill and seal, although it would be cool to experiment with different materials for the tube, since different material = different index of refraction. More on this in part 3 of the video. The tube should be long enough so that you can actually see the effect from the sides - our tube is 1 meter long, but you’d probably be able to see the effect with a ~0.5m tube. You also might want to make sure the tube is easy to open so you can clean it. See “Things to consider” for more. For the mini-demo, I just used a drinking glass! [FYI, the tube in this demo was custom-made for the MIT Physics Department. I’m not sure exactly where it came from or what type of glass it is exactly…] ⁃ A source of white, unpolarized light. We used a Dedo lamp, and if you make the demo as big as we did, you need a pretty powerful light source. For the mini-demo, I used my phone’s flashlight! ⁃ Two linear polarizing filters. You can get ‘em online pretty easily! Directions: ⁃ For our demo, we made a sugar solution of 300g of sugar per 400g of water. So, you should measure the volume of your tube and scale this ratio accordingly. ⁃ Boil the water and mix in the sugar until it’s dissolved. ⁃ Let the solution cool, then fill the tube with solution. Close up the tube. ⁃ Place the light source so it’s shining down the length of the tube, then place a filter between the light and the tube. ⁃ Place the other filter at the end of the tube. ⁃ Voila! You can rotate the first filter to see the whole spiral move up and down the tube, or you can rotate the last filter to see the color coming out of the end of the tube (and through the last filter) change. Things to consider: ⁃ You really want the tube to be clean before you start, since the solution can get moldy. If you look up close, you’ll actually see little floating things in the solution - those are some bacterial friends :) ⁃ We try to replace the sugar solution frequently so that the demo is clearer. ⁃ The shorter the tube is, the more concentrated you want the sugar solution to be in order to see a similar effect. ⁃ The amount of rotation of the polarization angle is proportional to the concentration of the solution (this is called the specific rotation!) ⁃ The light might get hot the longer you keep it on, so be careful! As always, observe sensible safety procedures. ⁃ You could do this with different sugars! Glucose would also rotate light to the right, but slightly less than sucrose. Fructose rotates light to the left! If you recreate this/do something else cool with it, I’d love to see!

[@Pranshu_Agrawal]

Only 47 Likes and no replies? Thanks for this and for taking the time to build this!

[@kredibled]

It’s likely borosilicate glass, most scientific glass is. You’re likely able to find a local glass blower who could make the fillable tube but the caps might require some thought. Could just superglue a small glass plate to the ends 🤔

[@Arc125]

Ordered filters to try this at home!

[@R23874]

Quinn, thank you so much for what you've built here. I greatly enjoyed this video, and it has rekindled my interest in studying optics.

[@Crushnaut]

Is LED light on your phone actually white light? When you split it, do you end up with the three specific wavelengths of light corresponding to wavelengths from each LED making up the white light? Either way, I think that would be a cool follow up to this. Use different light sources and see the effect. If you did use three wavelengths (rgb), how sharp would the drop from one colour to the next be? Would you get twisted lines around the tube for each colour or would there be areas dominated by each colour with gradient transitions?

[@cdenn016]

I've studied light/matter interactions for 20 years, published papers and such, and had never seen this before. I was pleasantly surprised! I wouldn't have thought you'd see light via the side. How great The more you learn the more you realize you know next to NOTHING

[@glenfoxh]

Gene Simmons from Kiss said next to the same thing about women. The more you think you know about women, the less you really know. And that is one guy, who has likely been with a lot of different women in his life.

[@91722854]

do you mind sharing what your research is in and your link on google scholar? I do research in optics

[@paveltroev8221]

the lamp is set off the axis

[@Eric-xh9ee]

I did research in optics. This should be obvious to you if this was your profession.

[@PowerhouseCell]

Love to see Grant branching out. He's truly made an impact on all online education, not just math! ❤

[@hanknew9685]

Omg yes this is getting into chemistry and enantiomers! Super excited to hear the math behind this

[@TheNameOfJesus]

The universe is made with complex math, so his expertise will be amazing if he breaks into physics and similar subjects.

[@theodorekim2148]

@PowerhouseCell Wait you're the guy who makes biology videos with Manim! Love your channel!

[@arvinderbali]

Basically, maths is behind all, so u understand math, u understand all.

[@TheNameOfJesus]

@@arvinderbali Maybe, but "maths" has been proven by Godel to be "incomplete" and therefore it seems unlikely that the universe is "incomplete." So there must be more to the universe than "maths."

[@3blue1brown]

Part 2 is available now: kzbin.info/www/bejne/l4m1hZawdrOof9Usi=m6DgY1ogMrwTRrUP Some viewers have asked about how to make this demo for themselves, and Quinn kindly wrote up the description below. Materials: ⁃ Table sugar (sucrose) ⁃ Water ⁃ A glass tube that you can fill and seal, although it would be cool to experiment with different materials for the tube, since different material = different index of refraction. More on this in part 3 of the video. The tube should be long enough so that you can actually see the effect from the sides - our tube is 1 meter long, but you’d probably be able to see the effect with a ~0.5m tube. You also might want to make sure the tube is easy to open so you can clean it. See “Things to consider” for more. For the mini-demo, I just used a drinking glass! [FYI, the tube in this demo was custom-made for the MIT Physics Department. I’m not sure exactly where it came from or what type of glass it is exactly…] ⁃ A source of white, unpolarized light. We used a Dedo lamp, and if you make the demo as big as we did, you need a pretty powerful light source. For the mini-demo, I used my phone’s flashlight! ⁃ Two linear polarizing filters. You can get ‘em online pretty easily! Directions: ⁃ For our demo, we made a sugar solution of 300g of sugar per 400g of water. So, you should measure the volume of your tube and scale this ratio accordingly. ⁃ Boil the water and mix in the sugar until it’s dissolved. ⁃ Let the solution cool, then fill the tube with solution. Close up the tube. ⁃ Place the light source so it’s shining down the length of the tube, then place a filter between the light and the tube. ⁃ Place the other filter at the end of the tube. ⁃ Voila! You can rotate the first filter to see the whole spiral move up and down the tube, or you can rotate the last filter to see the color coming out of the end of the tube change. Things to consider: ⁃ You really want the tube to be clean before you start, since the solution can get moldy. If you look up close, you’ll actually see little floating things in the solution - those are some bacterial friends :) ⁃ We try to replace the sugar solution frequently so that the demo is clearer. ⁃ The shorter the tube is, the more concentrated you want the sugar solution to be in order to see a similar effect. ⁃ The amount of rotation of the polarization angle is proportional to the concentration of the solution (this is called the specific rotation!) ⁃ The light might get hot the longer you keep it on, so be careful! As always, observe sensible safety procedures. ⁃ You could do this with different sugars! Glucose would also rotate light to the right, but slightly less than sucrose. Fructose rotates light to the left!

[@GooogleGoglee]

❤

[@richardsmith3021]

You should pin this.

[@d.mort.]

Hey there, I have a fundamental correction to the visuals in this video. The polarizing filters are oriented incorrectly. When the polarizer is vertical, it will transmit horizontal and reflect vertical. It’s a common misconception, if you would like to hear more I would be happy to share! (I am an electrical engineer with a passion for light haha) PS these are some amazing quality videos!

[@MatthiasBussonnier]

+1 to this, it's hard to test with visible light polarizers, but you can find educational material that does it with ~10cm wavelength. The idea is that the electrons can move in the direction of the line filter can move sufficiently along this direction that the dissipate the energy of the light polarized in the same direction right ?

[@GooogleGoglee]

@@MatthiasBussonnier but please, fix your English first, I am not understanding what you are asking exactly

[@Nighthawkinlight]

What a beautiful demo setup!

[@pierrefrebet5630]

Hello @Nighthawkinlight ! Love your channel !

[@tachoblade2071]

This is like being at the center of the research table of grant, listening to the ideas and just absolutely loving your time learning. This is exactly what college/higher education needs to be, collaborative on a world level. Ofcourse, i am only a viewer and not a collaborator, but it just feels insanely amazing to be able to listen to this information

[@TheArizus]

I 100% agree. As someone who's experienced maths olympiad camps my favourite part is the collaboration.

[@imad8903]

me too @@TheArizus

[@ManuelBTC21]

A landmark in science communication. Thank you and congratulations.

[@z0nx]

Just some lads making videos.......................................................................................................................... !

[@neiljudell1437]

I first saw this in 1973, in Walter Lewin's class on vibrations & waves at MIT. Damn, he was a great lecturer.

[@krissp8712]

True, and yet as great as he was, Lewin still turned out to be misusing his role like a creep in the end.

[@أحمدالدسوقي-ت9س]

Lewin is the best❤❤

[@itishappy]

I'm an optical engineer. I have solid intuitions about light. I still said "what?!" out-loud involuntarily when the lights dropped the first time. What an amazing video!

[@Serizon_]

it truly is an amazing video

[@userJohnSmith]

Right there with you (optical and opto-mechanical) the angle is weird. You just don't anticipate the "rotation" to be so slow...

[@KeithSmith42]

Any chance this will replace separate RGB diodes someday?

[@userJohnSmith]

@@KeithSmith42 Diodes replaced this actually. It's how LCD screens work.

[@KeithSmith42]

@@userJohnSmith So before RGB pixels were single tubes that used a filter to output the desired wavelength? I thought diodes replaced electron gun-based CRT screens

[@rex-up9ln]

Im a chemist. the dependence of angle of polarization by a chiral molecule on frequency of light is a very useful phenomenon which gives rise to cotton effect. Different molecules and even parts of a molecule have a different signature and thus the plot of angular dispersion vs. freq can help identify a molecule and functional or structural motifs. See Circular Dichorism spectroscopy

[@ENCHANTMEN_]

How does the chirality cause the twisting even though the molecules might be in any orientation? My intuition would be ones facing the opposite direction cause it to twist the opposite way, but clearly that's not the case here

[@tomc.5704]

@@ENCHANTMEN_ I'd love to know too -- I don't have the intuition for this at all. I'm trying to use my hands to represent chirality / circular polarization, with my thumbs representing the direction of travel and the curl of my fingers as the twist. It seems to make sense -- if you had an equal number of left hand and right hand chiral sugars (represented as both right and left hands with the thumbs pointing in the same direction) then the twist would cancel out. But it's also really easy to get the direction of twist to cancel out with one hand. Just turn your thumb to point the other direction. So what's different about pointing your hand in the opposite direction, and light hitting the sugar molecule from a different angle (ie the opposite side)?

[@MarkFord-k4z]

@@ENCHANTMEN_ The light does not just interact with a single molecule, so you can consider an average of all the molecules that the light interacts with being equivalent to averaging one molecule over all orientations. A chiral molecule averaged over all orientations cannot be superimposed a similar average for the other enantiomer - essentially the average is still chiral. Plane polarised light can be equivalently described as a balanced sum of right and left helically polarised light. The sample will interact differently with right and left helically (circularly) polarised light, which leads to the twist in the polarisation plane.

[@lesconrads]

The visualisations is on another level. I am incredibly impressed by how you can create these moving 3D animations to show super complicated concepts.

[@2Sor2Fig]

I remember learning about chirality in Chemistry 201. Most of the time I'm mostly along for the ride, but today it did feel intuitive. Learning from you has completely changed my view of mathematics, and I thank you for that.

[@dmaster254]

I'm at 2:04 right now in the video. prediction: the rainbow effect is a result of the sugar twisting light at different rates depending on the wavelength and then some escapes out the side after

[@shayantan243]

Sometimes, I wish I could just stay with Grant. I could do all his household chores, any work he would ask for and in return I would just learn from him, ponder about various questions in the universe, that's it and I'm just a happy man in this lifetime.....

[@ricardopinho630]

every video of this guy is a phd paper

[@astroboy0110]

So true 😅

[@sainadh7]

Beautiful experiment. As someone who worked in spectroscopy with chiral molecules, I must say this is a must demonstration in graduate classrooms. The experiment has refravtive index, polarisations, optical rotation due to chiral molecules, all playing their role simultaneously.

[@iainmackenzieUK]

I am a British Physics teacher working in China. I really appreciate the questions you ask and how you ask them/ explore ways of answering. gets to the core of what we are trying to encourage in our students. Great work

[@Ninjaeule97]

What made you move to China and work there?

[@李某-c6h]

I just learned about optical isomerism in A-level chemistry, and I'm not sure if it's related to this video.

[@iainmackenzieUK]

@@Ninjaeule97 The question really is "why would I ever move back?" :) Lovely students, interesting culture, many varied daily challenges, the money is pretty good.

[@iainmackenzieUK]

@@李某-c6h from what I recall of my chem I think it is very much linked...

[@Ninjaeule97]

@@iainmackenzieUK Glad that you enjoy your life there. I just haven't heared of many westerns move to Asia and choose China instead of one of the more democratic (and wealthier) countries in that region. So I had to ask.

[@abraxas2658]

The way you mentioned the answers without explicitly stating the methods really jogged my brain in an exciting way. Being able to think about twisting polarized light and angles of refraction together really gave me an AHA moment that saying "polarized light twisted at a sufficient angle from a surface would experience total internal reflection" just wouldn't have provided

[@bandana_girl6507]

Having played around with a lot of polarizing filters and looked through a ton of information about how it works with scattering, I have built up an intuition, and I am really excited to see how you make it easier to understand.

[@primenumberbuster404]

Steve Mould and Grant Sanderson has the best therapy voice of all time. ;)

[@claaaaams]

This is such a great video with the way it's setting up other discussions. Thank you for making physics accessible!!!

[@pamaramb]

I insist. This guy deserves the Nobel Prize in Education. And given that it doesn't exist, he also deserves it to be created.

[@davekwcc]

You have such a gift at explaining complicated subjects in interesting and engaging ways. Thanks for another excellent video!

[@BlueRaja]

9:45 "Our brains do something to make rainbows more vibrant than they really are" - actually it's because RGB isn't able to accurately represent intense colors. Only something like a third of the full human-vision chromaticity diagram can be represented using RGB.

[@TeaDrinkingGuy]

I have never seen this phenomenon before, which is a surprise because I love Steve’s videos. What a beautiful phenomenon that’s quite easy to replicate! Seeing this in science class as a kid would have seemed like magic.

[@rjkirkland8659]

This is better than Christmas...I love these very tangible, available examples of the complexity in the world and how it leads to fascinating experiences.

[@TheArizus]

A day with a 3blue1brown video is always great but a day with 2 uploads is exceptional!

[@Jacobk-g7r]

7:30 The reason its changed from the side is because we see what is absorbing the light/where it has frictions and thats why we see the specific waves bouncing off. Like as it scatters to the sides it still is in the direction of one way because its like a cone of light exiting the sides at different wavelengths but if you stand in front then youll see mostly white because majority is that way. The sugar and water make the light change, the change in frequencies keep each other in order like waves falling into troughs and lining up to where they match and then the light exiting the sides is always happening but our position is a factor when taking in the light and how much of each wavelength. The lens in is like one beginning so the change passing through is like majority the same and so the spin is the change and it only looks like a spin because the container and the exiting of light and the direction of change.

[@Taxodium-d2j]

Grant is trying something new, adding a bit of physics experiments to the recipe! Glad to see that the channel is looking towards a bit of variety. Not that the usual maths animations were bad, they were exceptional as ever, but this is great to exploit the viz towards more intertwined maths and physics, I love it. Keep it up, Grant!

[@PhysicswithElliot]

My jaw dropped when the spiral rainbow appeared. So freakin cool!

[@Roanmonster]

I am very much using this video in my classes, and my heart jumped a little when I saw you could do the demo yourself! We are adding optics back into our curriculum (highschool physics) and this will be so interesting to see

[@dubscheckum8246]

This is way beyond any highschool or AP course

[@zach4505]

So humble. Grant refers to the beautifully illustrated and insightful animations as mere cartoons

[@Inspirator_AG112]

This is the first physics video I have seen from this channel.

[@avoirdupois1]

What a fabulous demo. I would have loved to see this kind of display when I was taking college physics. The coordination between the rotation angle of the polarization, the brightness of the output color, all coordinated. Brilliant. This reminded me strongly of Feyneman's book, QED. Thanks Grant!

[@ManishaPatel-sl8dp]

This is a phenomenon occurring due to optical activity of sugar. Known as optical isomerism. If a compound is optically active , it will bend light to a certain degree. I don’t know why this wasn’t mentioned in the video.

[@marcusklaas4088]

I love your videos so much.

[@DrDeuteron]

0:18 Your linear polarizing filter has a little arrow (vector: direction, Y_(1, m )) showing the filter's alignment. It should be a double headed arrow b/c linear polarization is a tensor alignment (no direction, Y_(2, +/-2))

[@kfftfuftur]

When light is reflected / transmitted through a dielectric surface, the reflected and refracted light has different polarization. Specifically if the incidence angle happens to be the Brewster angle the difference in polarization is maximal. My guess is that because the ratio of reflection and transmission depends on the polarization of light, a polarized photon leaving the tube is more or less likely to be reflected on the surface of the tube depending on the polarization angle. So what we see when we look at the tube is that light will initially mainly leave the tube to the top and the bottom, but as the polarization the direction of the direction of the light leaving also changes.

[@gradientO]

3b1b physics videos are a delight to see

[@ltmcolen]

You have a gift and I am so grateful to watch this without paying you any money.

[@aryasaktiflister_aw]

I came to this video as someone who only vaguely remembers some HS optical physics so I didn't expect to be too intrigued. Instead, that animation and explanation moved mo to the verge of tears. So beautiful and curiosity-enabling

[@joelluth6384]

As always, your visualizations are beautiful in their elegance and simplicity, and really help to explain the concepts. Well done.

[@HomieSeal]

Oh nice, glad to see a new 3B1B video!

[@erics3367]

The color is something I haven't seen before. In physical chemistry we used the polarization to measure the concentration of the sugar with monochromatic light. In a way where we used the second folter after the tube and turned it to an angle where no light passed though, that gave us the angle the light turned and the concentration. ps we did this with sucrose which splits into fructose and glucose, inverted sugar, because it changes the angle. sp we measured the change of the angle to find the speed of the reaction. Would be interesting to see if this also affects the colors. My thesis would be, starting from sucrose, you would see the spiral slowly unwinding and then winding up in the other direction

[@logenninefinger3420]

Very nice topic! When Math meets Physics, things get interesting :)

[@kyoto9916]

This concept is pretty simple and just another beautiful display of the properties of the universe unfolding through our ideas for setups.

[@Erosis]

Holy mother of god this is great! Optics was one of my weakest subjects in physics. It felt like a lot of it was just "this is how it is." I'm so excited to follow this series. I feel embarrassed, though, that as someone with a degree in chemistry that I didn't even consider the chirality of the sugar. I've used polarimeters in lab! Duh!

[@danikahicks2210]

Neat. I think I get it. Polarizing the light makes it oscillate in one direction, but the rotation caused by the sugar makes it seems like the wavelength is increasing as the apex/nadir of the wave is rotated away/toward our eyes. Rotating either polarizing filter changes what orientation can pass thru; more vertical orientations are bluer, and as the wave rotates the wavelength appears from our eye's perspective to be longer, going thru the spectrum of visible light all the way to red. With a long enough tube (and powerful enough light source) the colors would shift from blue to red and back to blue over and over again along the length of the tube.

[@matheusrossetto5091]

I saw this video as one of the great documents some immense scientist from the 19th century left as his legacy. What youre doing is just incredible, a friend of mine and I literally chose our graduation courses (Mathematics for him, Mech. Eng. for me), greatly thanks to your influence during our high school. Thank you for your ridiculously amazing work!

[@GabrielPettier]

2:41 i half expected the colors in the tube to also move when moving the *other* filter, and i'm relieved, things are not yet incredibly weird, just, normal quantum weird.

[@gallium-gonzollium]

My feeling for question 3 could be due to the tube acting like a prism, scattering different light at slightly different directions, magnified by the lens effect of the cylindrical tube.

[@Jacobk-g7r]

6:20 The light is undergoing a change and spinning the filter lets one freq hit it perfectly due to the direction it lets waves through/accumulate on the filter. The sugar is a crystalline structure that makes light break into variations and those differences hit each other and keep each other in line like a rainbow and the arc of the water.

[@d.mort.]

I can’t find my old correction so I am reposting it here: Hey there, I have a fundamental correction to the visuals in this video. The polarizing filters are oriented incorrectly. When the polarizer is vertical, it will transmit horizontal and reflect vertical. It’s a common misconception, if you would like to hear more I would be happy to share! (I am an electrical engineer with a passion for light haha) PS these are some amazing quality videos! Edit 1 for clarification: An experiment I performed in undergrad involved a square with equally spaced vertical wires. We placed it between a transmitter and receiver (transmitting vertically polarized EM waves, maybe 3GHz but I don’t recall). When the wires aligned with the direction of polarization, the receiver signal dropped drastically and vice versa.

[@TheGreatAtario]

Isn't this just labeling the same thing with a different word?

[@ghislainbugnicourt3709]

That's interesting ! A quick convincing argument is that when the electrical field oscillates along the stripes it induces currents so the light is reflected like on a metal. If a polarizer has an arrow, what's the convention then ? Should it show the orientation of stripes or the polarization you obtain ?

[@d.mort.]

The arrow is a good indicator. I would say a vertical arrow to denote the direction it allows to pass through, but the lines should be perpendicular to that arrow. An experiment I did in undergrad involved a square with equally spaced vertical wires. We placed it between a transmitter and receiver (transmitting vertically polarized EM waves, maybe 3GHz but I don’t recall). When the wires aligned with the direction of polarization, the receiver signal dropped drastically and vice versa.

[@The_Canonical_Ensemble]

Who said that the vertical lines on the polarizers represented the orientation of the wires?

[@ghislainbugnicourt3709]

@@The_Canonical_Ensemble Well, that's how we perceived it at least. If I see a drawing of, say, a fence with vertical lines I'm not gonna guess that the physical planks they represent are horizontal.

[@CoverBydAn]

Love that steve just said no, he didnt think of the side view, rather than being defensive. And the 3b1b dude is really good looking! Probably because i already like him since he’s such a great communicator

[@worldhaseverything]

Thanks for lighting me to this new knowledge.

[@BradenDiaz-p8y]

Thanks it's amazing to see a live full visible light spectrum . A landmark in science communication. Thank you and congratulations..

[@160p2GHz]

As a polarimetrist-- this is so great! I'm so exited for these videos. You and Steve Mould are my favourite STEM communicators (and not just because you have both covered polarization multiple times). So exited for this series (And thanks to Quinn for coming up with this!)

[@Jacobk-g7r]

3:09 The light is being pushed against the sugar and the resistance when passing through, like the quantum tunneling like affect and the points the light change and come out different but the push is still there from the back and theres nowhere for it to go but the way being pushed so it spins. The direction of spin could be the light hitting the other wavelengths and keeps them in line where theyre supposed to be in connection to the colors around it.

[@taiconan8857]

QUINN BRODSKY DESERVES MAJOR PROPS! *What* an idea and solid execution of said demo! We likely wouldn't have had this video *without* her perspective. Thank you for returning to this in *force* 3B1B! And including Steve Mould was just icing on the cake~ 💎

[@aleksandarprodanov4454]

The intensity of the scattered light is dependent on the angle between the polarisation and the scattering direction. Even if we look at the tube from the side without moving, i.e. we don't change the direction of scattering we are looking at, the different polarisations of the different colors cause different scattering intensities in the same direction. This leads to some colors being scattered less and others more in our direction. This explains why we see color from the side. Even so, this only explains the observed horizontal color gradient. For diagonal stripes, we need vertical color gradient too. I think the reason for this, is the shape of the walls of the container. Since they are circular(the container is cylindrical), there is refraction from the walls, which effectively changes the direction from which we observe the scattering. This means that looking at the top or at the bottom, we see scattered light more in the upward vertical direction and more in the downward vertical direction, respectively. If this is true, the slope of the stripes may be greatly reduced if instead of cylindrical tube, a rectangular one is being used.

[@lotofAlexa1221]

Quantum physics lets go

[@geekjokes8458]

well, ultimately it should be explained by quantum mechanics but i think classical EM would be able to solve this or maybe it's necessary for the 3rd question, idk

[@dilation1057]

I don’t think quantum mechanics is REALLY required at a deep level most concepts such as chirality, refraction and polarisation can generally can be solved using the wave theory of light but for the third question i really doubt only optics can help. Overall speaking this problem really does some need of quantum mechanics.

[@iddomargalit-friedman3897]

I think classical EM is probably enough here

[@shrill8239]

​​@@geekjokes8458I am thinking it much more related to the optical properties of the material in consideration.... idk too?😅

[@stevenfallinge7149]

Not everything is quantum magic/woo/mystery or blah blah blah.* Too many people think "quantum" and start thinking like it's magic instead of actual science. *(of course, technically everything is quantum, a detail many also miss)

[@aSpyIntheHaus]

Holy Moley! This has Dunning Krueger'd me (You don't know, what you didn't know but we're confident that what you knew was sufficient until you learn something new) in such a wonderful way.

[@julianwalford]

Wow! I’ve loved your videos for quite some time, @3blue1brown, but I’ve never felt so excited as just now seeing this teaser. I did a PhD and postdoc in optics 20 years ago, in areas very close what you’ve shown here. This is the clearest and most beautiful video I’ve seen on this topic and I am already learning something I didn’t know before. Thank you so much for everything that you for maths and physics education around the globe!

[@yelmoralardclaw]

An interesting experiment to have would be to extend the tube even further, or set a second tube right next to it. My guess so far is: Q1: fotons cause induction magnetic field in the sugared water, which itself causes the different twist for different length of waves on a set distance; Q2: twisting rate depend on frequency, because the differet light waves carry different energy, therefore they cause different strengths of very localized induction magnetic fields; Q3: diagonal stripes might have something to do with how light scatters - as the light is polarized, any wavelength can escape the side of the side of the tube, but the least refraction will be achieved at a straight angle, and as the wavelengths have different twist period, they come in 90 degree angle to the viewed at different parts of the tube. In addition, I want to note how at 8:09 part of the tube turns black, as plenty of fotons seem to refract away from the viewer, therefore likely rendering the tube like it doesn't emit light.

[@AnExPor]

Yahhhh! Don't always like a cliffhanger but in this case, it means more 3B1B videos. Can't wait!

[@cameronclarke4062]

I did a presentation on this last term form my final physics project. There was not a whole lot of information online so it was very fun to figure out! Great job explaining it. I also didn’t think to look at the side of the tube when preforming the experiment.

[@stevepittman3770]

1:48 Ok, now I want a floor lamp filled with sugar water with a slowly rotating polarizing filter. Also I wonder what this would look like if you mirrored the far end so none of the light escapes out the top - would the light twist back to the original orientation and cancel out the apparent spiral of colors?

[@andrewharrison8436]

Yes please! As to your question - interesting - it would obviously make things brighter! I think each colour could be regarded as a bolt thread with a different pitch. Then the mirror would send each colour back down the same thread. It would then reassemble to one polarisation at the lamp end.

[@timp2751]

The light will continue to twist in the same direction (relative to direction of travel) so more likely to end up with a pattern of sort of rhombusy nature as the returning light and outbound light contribute different imbalanced spectra to what eventually makes it out of the side of the tube at any point

[@aritradhar1271]

the visualizations add so much depth to the explanations, actually awesome stuff.

[@PhilBoswell]

Subtitle nitpick at 6:04: s/weekly/weakly/ Very much looking forward to the next video 👍

[@MrFlaviojosefus]

@8:05 . Before seeing this video to the end, I wish to dare an explanation for the different colors one sees on the side of the cylinder. So, I think the scattering of the light in a determined direction by the sugar molecules is dependent on the polarization plane of the incident light. At each place on the way down the cylinder, the sugar molecules scatter more of the light (in the horizontal direction) that is vibrating in a determined plane. Since the different frequencies (colors) are twisted in different planes (at a place down the cylinder) more light of a determined frequency will be scattered in this place.

[@HeisenbergFam]

Didnt expect to see Blue's upload right on the start of September, cant wait to see another next month or in 2 months..

[@RubixB0y]

Comes across kinda passive-aggressive when there hasn't even been enough time to watch the video before posting that comment. I appreciate every video whenever it comes out because he puts quality first.

[@wayneyadams]

4:39 Refraction is also frequency dependent which causes separation of the colors when light passes through a lens. This creates color fringes in cheap telescope eyepieces.

[@cactuskiller3558]

Like for "Essence of topology"

[@ntq1ty]

I love how attentive and interested the cat is that's watching the home experiment at 6:45 😻

[@bryanchaves5691]

Great content. Keep up the insane work 😊

[@Dudleymiddleton]

What do you mean "Insane"? Do you actually know what that fucking word means? I am sick to the teeth of seeing and hearing it everywhere. Mental illness is a very serious subject and this is extremely offensive.

[@haraldmilz8533]

In fact, the answer to #3 ist pretty straightforward. Rayleigh scattering takes place at most at a 90° angle so that the colour of light seen in a specific direction depends in the current polarization angle. If you keep the first pol filter fixed, and you move your head from left to top to right in the direction of light, you will see the spiral colours moving as well. By the way the reason why a rainbow looks "deeper" with the naked eye compared to a photograph is probably the higher dynamic range of the eye.

[@d.lawrencemiller5755]

I was guessing that the curvature of the pipe has something to do with the diagonal stripes. I was wondering if it would look different in a square tube. Looking forward to understanding the rest of this video series so I can make a hypothesis about what a square tube full of syrup would look like

[@CYI3ERPUNK]

beautiful and fascinating ; well done ; looking forward to the rest of this indeed

[@tricanico]

classic 1000% likes to views ratio right now

[@fultzjap]

This is the best KZbin video I've seen in years. You REALLY nailed this

[@pythonboi5816]

Guys look a multi colored LED

[@kruksog]

I love this gathering of a couple of my very favorite KZbinrs.

[@aozorakei5288]

First

[@brainxyz]

Short wavelengths (blue) are slowed more sharply upon entering the sugar solution than are long wavelengths (red) due to the different refractive index, hence, colors look separated from the side of the tube.

[@carultch]

Could a material ever exist that has a uniform refractive index as a function of frequency? My name for such a material property, is isohertzic.

[@Lolwutdesu9000]

Just a small point to raise: linear polarisers actually allow light that is NOT parallel to the chains present in the polariser material, usually denoted as the straight lines drawn on a linear polariser. So if the polariser wants to allow vertically polarised light through, the lines should be aligned horizontally. I know it's a small menial detail, but it is important, as it can invoke incorrect assumptions about how light is polarised in the first place.

[@marcelofalcaodeoliveira4022]

Impressive experiment! So simple and so deep in knowledge! Mind blowing! Congratulations for the videos!

[@robertofontiglia4148]

pausing now (8:00) here's my guess. Reflection and refraction of light through a surface polarize the light that is reflected / refracted at that point, parallel to the surface and perpendicular to the propagation of the light. Because it's a cylinder, that means that from any angle, the light is polarized at a different orientation, but at further and further distances, the colours are "twisting" further and further; hence the helicoidal stripes... ?

[@Coolbeans_1999]

I really liked how this not so intuitive phenomenon was broken down and explained a lot more clearly then I think other communicators would.

[@eldarr0uge482]

I've studied that at uni, went through all the calculations and stuff and conducted the experiment (as a matter of fact it was my optics lab exam) so I knew exactly what was going to happen but I got to say, the visuals and animations are breathtaking !

[@tommyjs1992]

can i ask what type of light can someone make this with? im trying to replicate this and im missing this piece of information. Would a very brigth led light work?

[@AlphaPhoenixChannel]

yup - i'm hooked. I was torn between some kind of stress-in-the-glass-polarization, something having to do with polarization dependent reflection, and a vague un-thought-out concept of polarized scattering like the dark band 90 degrees from the sun... sounds like it's the third and i should think harder! can't wait to see the rest. In retrospect those first two don't work at all because the polarized twisting light is only traveling axially... until it scatters... hmmm...

[@AlphaPhoenixChannel]

oh shoot wait both parts out at once??? sweet

[@JTCF]

We were told about refraction that changes the direction of light depending on its frequency... Now we're getting to twist! This is going to be so cool!

[@fluiditynz]

That's pretty cool. took me just past half way, I paused for some seconds and the rest of it was confirmations but explained so much better than I could.

[@Kitastroboy]

These animations and narration are sublime.

[@ajreukgjdi94]

I wouldn't in a hundred years have guessed the effect would be so strong and so striking, but the overall "shape" of the effect was pretty close with what I had guessed. Well, in truth, I had 2 guesses. The first being nothing interesting, but my intuition told me when looking from the side, there was a good chance the lack of symmetry of the curvature of the tube with respect to the propagation vector would cause it to interact with different polarizations in a different way. Also, you wouldn't be asking us what we thought it would look like from the side unless there was an interesting phenomenon. In effect, looking down the barrel, we get a very symmetric outcome. If light is scattering from interactions with the tube itself, any differences will cancel out because if you rotate your view so a given polarization is vertical, the image of the circular cross section does not change. But when looking from the side, you simply cannot stand still and rotate your view so the polarization at a given distance is vertical. What you would have to do is rotate your entire body around the central axis of the tube. And that's essentially what you're doing by rotating the filter. Instead of rotating yourself, you're equally rotating all the polarizations. So there is some asymmetry which, in effect, causes the tube itself to have a polarizing effect. Of course when you frame the question as "why isn't it white when there are no other polarizing filters" becomes a question with a faulty premise. There is a filter in disguise. Then the diagonal stripes are just whichever polarization(s) are lined up well to scatter in the direction of the observer. Move the observer, and you change the color. I'm not sure if those are aligned with the observer or perpendicular, or somewhere in between. My git would say it has to be aligned or perpendicular, but I've no rationale why. Certainly, it won't be a perfect cutoff, like the imbalanced dot product effect you showed looking through the end filter, there will be a range of polarizations that are well aligned. That would account for the "width" of the stripes. Of course the stripes are smoothly changing color, but you get what I mean. And the spiraling nature of the polarization described in Steve's video means as the light moves further from the source, new colors will enter and leave that band of well aligned polarizations lending to the diagonal nature of the stripes. Just because my intuition seems to agree doesn't mean it's actually a correct mental model, so I'm interested in tbe next video. It's been a long time since I took E+M fields or optics in college, so I hope I still remember some of the math.

[@Toxodos]

in the subtitles it says "coloours which end up more perpendicular to the filter, pass through only very weekly" and I'd like to thank you for taking all this time to do this experiment

[@ZbyszekJot]

Man, you deserve a Nobel just fot those animations!

[@tarikb.9497]

I am watching you guys because of 2 things : "WOW !" and "WHY ANYHTING ?!" and I beieve that this is the essence of science...

[@priyanshshrivastava8842]

such a joy that this dropped when i am studying about optical activity in my chemistry lectures.