(gentle music) - I'm here in L.A. with my friends, William Osman and Allen Pan.

You guys are gonna do a little bit of art here.

You just decorate it with strips of tape, this real cheap clear tape.

- Okay.

Do you want lots of overlapping or?

- [Physics Girl] Up to you.

- I'm doing a lot of overlapping.

- Mine looks like a pretzel stick with really big salt crystals.

(Physics girl giggles) - I don't know what's gonna happen here but now you're making me excited.

- So this piece has Allen's hair on it.

- [Physics Girl] Where's the hair?

- It's right... You see the long bubble?

(laughs) - Hermione is not happy with Ron and Harry right now.

(Allen and William laughing) Go ahead and look through your art pieces at the light-ish.

How's it look?

- It just looks like tape.

And I can see there's a bunch of fingerprints cause I was not-- (William speaking drowns out Allen) - There's like little circles.

- [Physics Girl] Okay, nothing.

These are polarized sheets.

Now, hold this behind it.

Put this one in front of it.

- Whoa!

- Whoa!

There's colors!

- [Physics Girl] Now, turn it.

Turn the... - Oh!

The colors are changing.

- [Allen] It's like an ugly-- - [William] Whoa!

- [Allen] It's like an ugly wreath.

(Physics girl laughing) - [William] That's beautiful.

It's not ugly.

- I love this demo.

Okay, just to clarify, you can do this demo in a few different ways.

Behind the plastic with the tape, you can either put an LCD screen or one of the polarized sheets.

And then in front plastic, you either wear polarized glasses to look at it or you can use a second polarized sheet.

Amazing!

You don't have a clear plastic fork, do you?

- [Allen] I may.

- [William] That's really awesome.

- Dang!

- That is so cool.

- [Allen] Wait.

All right, we got knives.

I don't have forks though.

Whoa!

How could you tell what's going on?

- [William] Well, there's some stuff going on here too.

- [Physics Girl] What!

That is so cool.

- [William] Little plastic Petri dish.

- [Allen] Whoa!

(Allen gasping drowns out Physics girl) - Oh, that's cool.

- [Allen] I love how you can see where all the stress concentrations end up.

This is like relatively like gradual transition and then it just gets crazy by the process.

- [William] Right there.

- Yeah, engineers actually use a similar setup of polarizers to look for stress in material samples.

It's called a polarimeter.

Okay, so how is it that you can get these crazy, beautiful rainbow colors when all of these things are either clear or gray?

Let's start out by talking about what these are.

Polarizers.

The lenses in your polarized glasses, those are polarizers.

They're filters that let through light with a certain orientation.

I'm going to ask you very politely to pretend please that this rope is a light wave.

(playful piano music) Except light is not a rope that waves up and down, it's an electric field going up and down with a perpendicular magnetic field also waving.

So light waves can go up and down but they can all also go side to side and diagonally.

And the direction that the electric field waves is called the polarization of the light wave.

So rope, not light, please pretend.

Vertically polarized, horizontally polarized, diagonally polarized.

Light reflecting off of puddles and oceans is often horizontally polarized, depends on the angle, depends on the flatness of the water, but keep it in mind.

I should probably also mention before someone in the comment scolds me for not mentioning that there's also circularly polarized light and elliptically polarized light, but those come from adding different linearly polarized light waves together, like this horizontal and this vertical light that are out of phase.

And then there's unpolarized light, which is most light.

And that's just a bunch of random polarizations added together.

Great.

Okay.

Now, polarizing filters only let through light of a certain polarization.

For example, if this is a vertically polarized filter, it'll only let through light that is vertically polarized and it'll block the rest of the light.

Interestingly, the light that has a diagonal polarization, some of that will get through because it has a component in the vertical direction.

So how much of that light gets through depends on how vertical the diagonal angle is.

And so now if you wanna block light reflecting off of water, you should be blocking the horizontal polarization.

So you use a polarizing filter that's oriented vertically.

That's what manufacturers do with polarized sunglasses.

They orient their polarized lenses so they're blocking horizontally polarized light.

So if you have polarized sunglasses, you can turn them side to side to make sure that they're blocking light reflected off of flat water surfaces.

Also, light from LCD monitors is polarized because LCD screens have a polarizing filter in front of them.

So light from your laptop screen is probably polarized.

And though some animals can detect this polarization with their eyes, us humans can't do it without the aid of sunglasses.

(playful music) Or can we?

- Physicist on biology, hm?

Human can actually perceive the polarization of lights.

And you will also during this video.

Maybe.

But let's first understand how and why some animals perceive the polarization of light.

The natural world is full of flat surfaces that can linearly polarize light, such as wet leaves or bodies of water.

The scattering of light in the atmosphere or underwater can also partially polarize light.

And all this information is used by animals for a variety of reasons.

For example, polarization is perpendicular to the direction of sunlight.

So birds, insect and even some mammals use it for orientation when they can't see the sun directly.

Cuttlefish, who have this amazingly subtle perception of polarization, use it to prey on silver fish, even with their light reflecting camouflage.

And other predators are even able to detect the most transparent animals, thanks to polarized light.

This is because light becomes polarized when it's passing through the animal's tissues and the poor prey becomes visible to the predator.

So the point is polarization of light is an extra piece of information that animals make use of in many different ways.

But how about humans?

Well, first let's try something.

Get this video full screen and look at it.

You can also do that with any blank image if you don't trust me.

Now, tilt your head from left to right slowly while looking at your LCD screen.

- [Physics Girl] Tell me if you see anything.

- It seems like there's spots.

- [Physics Girl] What kind of spots?

- Like there's a spot in the middle and then it kind of looks like lines, like diagonal lines.

- I don't see it.

- [Physics Girl] What color?

Is there a color?

- Just kinda yellow, I don't know.

- Oh, okay.

Maybe it's just in the middle, cause I was looking in the middle.

Yeah.

There's some yellowness, I guess.

- But it's like streak.

- It looks like this.

- [Physics Girl] Okay.

- It looks like blotches out like that.

- It's-- - [Leo] If your screen is cleaner than Physics Girl's, a very faint yellowish and blue pattern of the size of your thumb should appear.

If you don't see it or if you're not really sure of what you are looking for, the pattern should look like this.

- Yeah, like a... - That's what it looks like, yeah.

- [Physics Girl] That's what it looks like?

- Yeah, but oh, is there supposed to be blue too?

I didn't notice any blue.

But yeah, I noticed the squish yellow stain.

- This pattern is called the Haidinger's brush and it is thought to be caused by the presence of yellow pigments arranged circularly in the retina.

So there you go.

Yes, human can see the polarization of light.

- What!

Some humans can see polarized light.

That is so cool.

Biology earns some cred from the Physics Girl channel today.

Okay.

Back to the final little mystery of our demo.

Why the crazy colors?

So the key is the tape.

Whatever of the material the tape is made of actually twists or turns the polarization of light as it's passing through.

It twists it differently for different colors of light.

So you get the polarized light passing in, it's turned and twisted, and then it hits the second filter.

Only light of certain colors has been twisted to the right amount, twisted to the right angle so that it can pass through the second polarizing filter and most of the rest of the colors are blocked.

So you get preferentially some colors passing through and not others as much.

Now, if you turn the second polarizing filter more then it's different colors that have twisted the right amount to get through.

And the result is fun up.

It's cool.

Thank you so much to Leo who came in and offered that super interesting information.

Also thank you to Allen Pan and William Osman who helped out.

Their channels are awesome.

They do crazy, funny engineering project.

Thank you for watching and... - Happy physicsing.

- Happy physicsing.

- I'm gonna go take a nap.