Thanks to brillant.org for supporting PBS Digital Studios.
I'm here in L.A. with my friends William Osman and Allen Pan.
You guys are going to do a little bit of art here.
You just decorate it with strips of tape--
this real cheap, clear tape.
OK.
Do you want lots of overlapping or?
PHYSICS GIRL: It's up to you.
I'm doing a lot of overlapping.
Mine looks like a pretzel stick with really big salt
crystals.
[GIGGLING]
WILLIAM OSMAN: I don't know what's going to happen here.
But now you're making me excited.
So this piece has Allen's hair on it.
PHYSICS GIRL: Where's the hair?
ALLEN PAN: It's right-- you see the long bubble?
Hermione is not happy with Ron and Harry right now.
[LAUGHTER]
Go ahead and look through your art pieces at the light "ish."
How's it look?
Well, this looks like tape, and I can see there's a bunch
of fingerprints because I [INTERPOSING VOICES]..
There's like little circles.
PHYSICS GIRL: OK.
Nothing.
These are polarized sheets.
Now hold this behind it.
OK.
PHYSICS GIRL: Put this one in front of it.
Whoa!
There's colors!
PHYSICS GIRL: Now turn it.
Like, turn the--
WILLIAM OSMAN: Oh!
The colors are changing.
ALLEN PAN: It's like an ugly-- it's like an ugly wreath.
[LAUGHTER]
WILLIAM OSMAN: That's beautiful!
It's not ugly.
I love this demo.
OK.
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 of the 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 PAN: I may.
WILLIAM OSMAN: That's really awesome!
Dang!
PHYSICS GIRL: This is so cool.
ALLEN PAN: Wait.
All right, we've got knives.
I don't have forks though.
Whoa!
How could you tell, like, what's going on?
WILLIAM OSMAN: Well, there's some stuff going on here too--
What?
That is so cool.
--a little plastic petri dish.
Whoa!
PHYSICS GIRL: That is cool.
ALLEN PAN: I love how you can see where all the stress
concentrations end up, like this is a relatively
like gradual transition, and then it
just gets crazy right there.
WILLIAM OSMAN: Right there.
PHYSICS GIRL: Yeah.
Engineers actually use a similar set up of polarizers
to look for stress in material samples.
It's called a polarimeter.
OK.
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 are 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.
Except light is not a rope that waves up and down.
It's an electric field going up or down
with a perpendicular magnetic field also waving.
So light waves can go up and down.
But they can 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.
It depends on the angle and depends on the flatness
of the water, but keep in mind.
I should probably also mention-- before someone in the comments
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 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.
OK.
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 want to 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 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.
Or can we?
Physicist and biology--
humans can actually perceive the polarization of light,
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 under water
can also partially polarize light.
And all of this information is used by animals
for a variety of reasons.
For example, polarization is perpendicular to the direction
of sunlight.
So birds, insects, 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 silverfish,
even with their light-reflecting camouflage.
And other predators are even able to detect
the most transparent animals thanks to polarized lights.
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 are 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?
It's just kind of yellow.
I don't know.
Oh.
OK.
Maybe it's just in the middle because I
was looking in the middle.
Yeah.
There's like some yellowness, I guess-- like a streak.
I think it's like this.
It looks like it blotches out like that.
LEO GRASSET: 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
sure of what you're looking for, the pattern
should look like this.
Yeah, like a--
ALLEN PAN: That's what it looks like, yeah.
PHYSICS GIRL: That's what it looks like?
ALLEN PAN: Yeah.
WILLIAM OSMAN: Oh, is there supposed to be blue too?
I didn't notice any blue?
But yeah, I noticed the squished yellow stain.
LEO GRASSET: This pattern is called the haidinger's brush,
and it is thought to be caused by the presence
of yellow pigments arranged similarly in the retina.
So there you go.
Yes, human can see the polarization of flight.
What?
Some humans can see polarized light.
That is so cool.
Biology earns some cred from the Physics Girl channel today.
OK.
Back to the final little mystery of our demo.
Why the crazy colors?
So the key is--
the tape.
Whatever 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 fan--
it's cool.
Thank you so much to Leo, who came in and offered
that super interesting information.
You can check out his channel.
It's called Dirty Biology, and I linked to it
in the description.
It's in French, FYI.
Also, thank you to Allen Pan and William Osman who helped out.
Their channels are awesome.
They do crazy, funny engineering projects.
I've linked to their channels in the description as well.
Thank you for watching, and--
happy physicsing.
I'm going to go take a nap.
Just kidding!
I'd like to thank brilliant.org for supporting PBS Digital
Studios.
Brilliant.org is a unique learning site
that offers practice problems on all kinds of things.
You could check out the symmetry lesson
to learn about snowflakes, and then challenge yourself
with exercises.
For science enthusiasts like us, brilliant.org
offers interactive challenge problems,
and then you can learn the underlying concepts.
To check out brillant.org, go to brilliant.org/physicsgirl.
I'll put that link in the description,
and the first 200 to click will get 20% off an annual premium
subscription.
For vyer-- va-ri-ety--
variety.
PHYSICS GIRL: There's no way I'll use these in the bloopers
at the end-- no way at all.
[LAUGHTER]
We should switch.
Let's try something simple.
[SPEAKING FRENCH]
Something simple?
[SPEAKING FRENCH]
[MUSIC PLAYING]