Polarized light in sugar water | Optics puzzles 1

Shining polarized light into sugar water reveals diagonal stripes of color. Why?
An equally valuable form of support is to simply share the videos.

Thanks to Quinn Brodsky for setting up the demo and to the MIT Physics Instructional Resources Lab for their help and materials, especially Josh Wolfe and Caleb Bonyun.

Thanks to these viewers for their contributions to translations
German: Josh, jns-v
Hebrew: Omer Tuchfeld
Hungarian: MrExpert
Indonesian: akhyarr
Italian: Gabboronco
Korean: @tebaioioo, Juhyung
Persian: ah.202
Russian: fedor, ilevinson
Spanish: Marcelo Lynch

------------------

These animations are largely made using a custom Python library, manim. See the FAQ comments here:

All code for specific videos is visible here:

The music is by Vincent Rubinetti.

------------------

3blue1brown is a channel about animating math, in all senses of the word animate. If you're reading the bottom of a video description, I'm guessing you're more interested than the average viewer in lessons here. It would mean a lot to me if you chose to stay up to date on new ones, either by subscribing here on YouTube or otherwise following on whichever platform below you check most regularly.

[SteveMould]

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

[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!

[cdenn]

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

[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!

[PowerhouseCell]

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

[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!

[ManuelBTC]

A landmark in science communication. Thank you and congratulations.

[tachoblade]

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

[neiljudell]

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

[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.

[rex-upln]

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

[ricardopinho]

every video of this guy is a phd paper

[SorFig]

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.

[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

[sainadh]

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.

[matheusrossetto]

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!

[ntqty]

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

[bandana_girl]

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.

[abraxas]

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

[claaaaams]

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