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! 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] [SPEAKING FRENCH] [MUSIC PLAYING]
