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- Let's talk about polarization of light.

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We know what light waves are;
they're electromagnetic waves.

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So they're made out of electric fields.

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And that's not good enough.

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We know there's not just electric fields.

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That couldn't sustain itself.

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There's got to be magnetic
fields there, as well,

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that are changing.

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Those are perpendicular, so
you can kind of draw them.

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It's hard, on something two-dimensional,

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but you can kind of imagine those

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looking something like this.

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And those magnetic fields
would point at a right angle

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to the electric fields.

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But this gets really
messy if I try to draw

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both the electric and magnetic
fields at the same time.

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So we're going to leave
the magnetic fields out.

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It's often good enough to
just know the direction

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of the electric field when we
focus on the electric field.

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So what does polarization mean?

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Polarization refers to the
fact that, if this light ray

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was heading straight toward
your eye, or a detector,

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over here, what would you see?

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Well, if I draw an axis over here,

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and this point here, in the
middle, this is this line --

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so imagine we're looking
straight down that line --

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and then up and down is up and down,

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and then left and right,

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that direction I have the magnetic field,

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would be this way and that way.

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What would my eye see?

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Well, my eye's only going
to see electric fields

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that either point up or
electric fields that point down.

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They might have different
values, but I'm only going to see

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electric fields that point up or down.

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Because of that, this
light ray is polarized.

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So polarized light is light
where the electric field

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is only oscillating in one direction.

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Up or down, that's one
direction -- vertically.

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Or it could be polarized horizontally.

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Or it could be polarized diagonally.

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But either way, you could
have this wave polarized

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along any direction.

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I mean, a light ray
like this, if we had it

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coming in diagonal,

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this light ray that's
oscillating like this,

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where the electric field
oscillates like that,

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that also polarized.

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These are both polarized because
there's only one direction

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that the electric field is oscillating in.

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And you might thing, "Pff,
how could you ever have

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"a light ray that's not polarized?"

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Easy.

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Most light that you get is not polarized.

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That is to say, light
that's coming from the sun,

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straight from the sun --
typically not polarized.

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Light from a lightbulb, an
old incandescent light bulb,

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this thing's hot.

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You can get light
polarized in any direction,

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all at once, all overlapping.

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So if we draw this case for a light bulb,

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just a random incandescent light bulb,

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you might get light, some of
the light, hitting you eye,

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you can get some light
that's got that direction,

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you got light that's got this direction,

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you got light in all these directions

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at any given moment.

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I mean, you'd have to add
these up to get the total,

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and they might not all be the same value.

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But what I'm trying to say
is, at any given moment,

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you don't know what direction
the electric field's

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going to be hitting your
eye at from a random source.

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It could be in any direction.

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So this is not polarized.

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This diagram represents
light that is not polarized.

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At some point, the field
might be pointing this way,

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at some later point it's
this way; it's just random.

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You never know which
way the electric field's

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going to be pointing.

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Whereas these over here,
these are polarized.

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So how could you polarize this light?

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Let's say you wanted
light that was polarized.

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You were doing an experiment.

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You needed polarized light.

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Well, that's easy.

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You can use what's called a polarizer.

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And this is a material
that lets light through,

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but it only lets light
through in one orientation,

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so you're going to have a
polarizer that, for instance,

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only lets through
vertically polarized light.

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So this is a polarizer.

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These are cheap:

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thin, plastic, configured in a way so that

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it only lets light through
that's vertically polarized.

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Any light coming in here
that's not vertically polarized

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gets blocked, or absorbed.

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So what that means is, if
you used this polarizer

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and held it in between your
eye and this light bulb,

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you would only get this light.

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All the rest of it would get blocked.

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Or you could just rotate this thing

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and imagine a polarizer
that only lets through

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horizontal light.

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Now it would only let through
light that was this way,

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and so you would only get
this part of the light.

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Or you could just orient
it at any angle you want

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and block everything but the certain angle

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that this polarizer is defined
as letting light through.

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So you can do this.

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And once you hold this up,
you get polarized light,

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light that's only got one orientation.

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So that's what polarization means.

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But why do we care about polarization?

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Well, let me get rid of this for a minute.

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You've heard of polarized sunglasses.

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So imagine you're standing near water,

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or maybe you're standing on ice or snow

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or something reflective.

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There's a problem.

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Say the sun's out.

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It's shining.

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It's a beautiful day -- except
there's going to be glare.

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Let's say you're looking down at something

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here on the ground.

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It's going to get light
reflecting off of it from just ...

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you know, light's coming
in from all direction.

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But it also gets this
direct light from the sun.

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So it gets light from
reflected off the clouds

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and whatever, whatever's
nearby, ambient light.

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And there's also this direct sunlight.

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That's harsh.

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If that reflects straight
up to your eye, that hurts.

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You don't like that.

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It blocks our vision.

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It's hard to see, it's glare.

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We don't want this glare.

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So what can we do?

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Well, it just so happens
that, when light reflects

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off of a surface, even
though the light from the sun

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is not polarized, once it
reflects, it does get polarized

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or at least partially polarized.

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So this surface here,
once this light reflects,

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it's coming in at all orientations.

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You got electric field ...

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you never know what electric
field you're going to get

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straight from the sun.

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And when it reflects,
though, you mostly get,

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upon reflection, the
direction of polarization

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defined by the plane of
the surface that it hit.

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So because the floor is horizontal,

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when this light ray hits
the ground and reflects,

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that reflected light
gets partially polarized.

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This horizontal component
of the electric field

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is going to be more present
than the other components.

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Maybe not completely.

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Sometimes it could be.

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It could be completely polarized,

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but often it's just partially polarized.

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But that's pretty cool,
because now you know

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what we can do.

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I know how to block this.

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We should get some sunglasses.

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We put some sunglasses on
and we make our glasses

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so that these are polarized.

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And how do we want these polarized?

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I want to get rid of the glare.

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So what I do is, I make sure my sunglasses

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only let through
vertically polarized light.

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Here's some polarizers.

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That way, a lot of this glare gets blocked

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because it does not have
a vertical orientation,

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it has a horizontal orientation.

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And then we can block it.

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So that's one good thing that
polarization does for us,

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and understanding it,
we can get rid of glare.

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Also, fishermen like it because,
if you're trying to look

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in the water at fish, you want
to see in through the water,

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you want to see this light
from the fish getting to you.

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You don't want to see
the glare off of the sun

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getting to you.

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So polarized sunglasses are useful.

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Also, we can play a trick on our eye,

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if we really wanted to.

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You could take one of these, make one eye

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have a vertical orientation
for the polarization,

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have the other eye with a horizontal ...

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and you're thinking, "This is stupid.

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"Why would you do this for?"

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"This eye's going to get a lot of glare."

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We wouldn't use these outside,

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when you're, like, skiing or fishing,

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but you could play a trick on your eyes

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if you went to the movies and
you went and watched a movie.

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Well, the reason our eyes see 3D is

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because they're spaced a little bit apart.

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They each get a different,
slightly different image.

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That makes us see in 3D.

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We can play the same trick on our eye

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if we have the polarization like this.

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If light, if some of the light
from the movie theater screen

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is coming in with one polarization,

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and the other light's coming
in with the other polarization,

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we can send two different
images to our eyes

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at the same time.

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If you took these off,
it'd look like garbage

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because you'd be getting both of these

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slightly different images,
it'd look all blurry.

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And it does.

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If you take off your 3D
glasses and look at a 3D movie,

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looks terrible, because now

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both eyes are getting both images.

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But if you put your glasses back on,

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now this eye only gets the orientation

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that it's supposed to get, and this eye

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only gets the orientation
that it's supposed to get,

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and you get a 3D image.

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So it's useful in many ways.

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Let me show you one more thing here.

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Let's come back here.

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This light was polarized vertically.

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So that's called linear polarization.

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Any time ...

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Same with these.

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These are all linear polarization

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because, just up and down,
one linear direction,

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just diagonal.

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This is also linear.

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All of these are linear.

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You can get circular polarized light.

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So if we come back to here,

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we've got our electric field
pointing up, like that.

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Now let's say we sent
in another light ray,

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another light ray that
also had a polarization,

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but not in this direction.

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Let's say our other light
ray had polarization

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in this direction, so it looks like this,

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kind of like what our magnetic
field would have looked like.

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But this is a completely
different light ray

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with its own polarization
and its own magnetic field.

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So we send this in.

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What would happen?

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Well, at this point, you'd
have a electric field

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that points this way.

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At this point, you'd have a electric field

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that points that way.

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What would your eye see
if you were over here?

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Let's see.

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If I draw our axis here.

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All right, when this point
right here gets to your eye,

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what am I going to see?

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Well, I'm going to have a light ray

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that's one part of a light ray.

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One component points up.

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That's this electric field.

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One component points left.

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That's this electric field.

257
00:10:24,100 --> 00:10:28,434
So the total, my total electric
field, would point this way.

258
00:10:28,434 --> 00:10:29,800
I could to the Pythagorean theorem

259
00:10:29,800 --> 00:10:31,400
if I wanted to figure out the size of it,

260
00:10:31,400 --> 00:10:33,200
but I just want to know
the direction for now.

261
00:10:33,200 --> 00:10:34,633
And then it gets to here, and look at it:

262
00:10:34,633 --> 00:10:36,114
they both have zero.

263
00:10:36,114 --> 00:10:38,269
This light ray has zero electric field,

264
00:10:38,269 --> 00:10:39,700
this one has zero electric fields.

265
00:10:39,700 --> 00:10:41,200
So then it'd just be at zero.

266
00:10:41,200 --> 00:10:42,900
Now what happens over here?

267
00:10:42,900 --> 00:10:44,377
Well, I've got light.

268
00:10:44,377 --> 00:10:49,377
This one points to the right
at that point, this pink one,

269
00:10:50,800 --> 00:10:53,866
and then this red one
would be pointing down.

270
00:10:53,866 --> 00:10:55,633
So what would I have at that point?

271
00:10:55,633 --> 00:10:58,539
I'd have light that went this way,

272
00:10:58,539 --> 00:11:01,067
and it would just be
doing this over and over.

273
00:11:01,067 --> 00:11:01,967
It would just be ...

274
00:11:01,967 --> 00:11:04,668
I'd just have diagonally polarized light.

275
00:11:04,668 --> 00:11:06,667
This isn't giving me anything new.

276
00:11:06,667 --> 00:11:07,756
You might think this is dumb.

277
00:11:07,756 --> 00:11:08,433
Why do this?

278
00:11:08,433 --> 00:11:10,566
Why send in two different waves

279
00:11:10,566 --> 00:11:12,867
to just get diagonally polarized light?

280
00:11:12,867 --> 00:11:14,975
I could have just sent in one wave

281
00:11:14,975 --> 00:11:18,133
that was diagonally polarized
and got the same thing.

282
00:11:18,133 --> 00:11:21,533
The reason is, if you
shift this purple wave,

283
00:11:21,533 --> 00:11:24,833
this pink wave, by 90 degrees of phase,

284
00:11:24,833 --> 00:11:29,500
by pi over two in phase,
something magical happens.

285
00:11:29,500 --> 00:11:34,366
Let me show you what happens
here, if we move this to here.

286
00:11:34,366 --> 00:11:39,292
Now we don't just get diagonally
linear polarized light.

287
00:11:39,292 --> 00:11:40,275
What we're going to get is ...

288
00:11:40,275 --> 00:11:41,500
Let me get rid of this.

289
00:11:42,100 --> 00:11:46,833
Okay, so we start off with red, right?

290
00:11:46,833 --> 00:11:49,596
The red electric field points up, and then

291
00:11:49,596 --> 00:11:53,133
this pink wave's electric
field is zero at that point.

292
00:11:53,133 --> 00:11:54,052
So this is all I have.

293
00:11:54,052 --> 00:11:57,066
My total electric field would just be up.

294
00:11:57,066 --> 00:11:58,133
I'm going to draw it right here.

295
00:11:58,133 --> 00:11:59,604
The green'll be the total.

296
00:11:59,604 --> 00:12:01,766
Now I come over to
here, and at this point,

297
00:12:01,766 --> 00:12:04,433
there's some red electric
field that points up,

298
00:12:04,433 --> 00:12:07,533
but there's some of this
other electric field

299
00:12:07,533 --> 00:12:08,900
that points this way.

300
00:12:08,900 --> 00:12:12,067
So I'd have a total electric field

301
00:12:12,067 --> 00:12:13,667
that would point that way.

302
00:12:13,667 --> 00:12:15,766
And then I get over to here,

303
00:12:15,766 --> 00:12:19,033
and I'd have all of the electric
field from the pink one,

304
00:12:19,033 --> 00:12:20,600
none from the red one.

305
00:12:20,600 --> 00:12:22,733
It would point all left then.

306
00:12:22,733 --> 00:12:23,934
Look what's happening.

307
00:12:23,934 --> 00:12:27,067
The polarization of this
light, if I shift this,

308
00:12:27,067 --> 00:12:29,233
if I'm sitting here, looking with my eye,

309
00:12:29,233 --> 00:12:30,904
as my eye receives this light,

310
00:12:30,904 --> 00:12:35,066
I'm going to see this light
rotate its polarization.

311
00:12:35,066 --> 00:12:37,033
The polarization I'm going to notice

312
00:12:37,033 --> 00:12:39,767
swings around in a circular pattern.

313
00:12:39,767 --> 00:12:44,767
And because of this, we call
this circular polarization.

314
00:12:45,800 --> 00:12:48,372
So this is another type of polarization,

315
00:12:49,710 --> 00:12:54,710
where the actual angle of
polarization rotates smoothly

316
00:12:55,300 --> 00:12:57,400
as this light ray enters your eye.

317
00:12:57,400 --> 00:12:58,966
And you know what?

318
00:12:58,966 --> 00:13:00,441
Er, drrr ...

319
00:13:00,441 --> 00:13:03,368
All right, actually, I sent
you to receive this one first.

320
00:13:03,368 --> 00:13:04,233
That makes no sense.

321
00:13:04,233 --> 00:13:06,366
You're going to receive the
ones closes to you first

322
00:13:06,366 --> 00:13:08,567
in this light ray going this way.

323
00:13:08,567 --> 00:13:11,433
So you'd actually receive
this one first, then that one,

324
00:13:11,433 --> 00:13:12,633
then this one, then this one.

325
00:13:12,633 --> 00:13:15,134
Because of that, you wouldn't see this

326
00:13:15,134 --> 00:13:17,734
going in a counterclockwise
way, you'd see this

327
00:13:17,734 --> 00:13:21,374
going in a clockwise
circularly polarized way.

328
00:13:21,374 --> 00:13:22,533
Sorry about that.

329
00:13:22,533 --> 00:13:24,133
You might think, "Okay, why?

330
00:13:24,133 --> 00:13:26,648
"Why even bother with
circular polarization?"

331
00:13:26,648 --> 00:13:29,100
Well, I kind of lied earlier.

332
00:13:29,100 --> 00:13:31,070
Turns out, in the movie theater example,

333
00:13:32,146 --> 00:13:35,000
they don't actually do
it like this, typically.

334
00:13:35,000 --> 00:13:37,300
Oftentimes in the movie theaters,

335
00:13:38,166 --> 00:13:41,237
we don't have just linearly
polarized sunglasses.

336
00:13:41,237 --> 00:13:42,634
This would be a problem
because, when you look

337
00:13:42,634 --> 00:13:43,833
at the movie theater screen,

338
00:13:43,833 --> 00:13:47,200
and if you were to tilt your
head just a little bit ...

339
00:13:47,200 --> 00:13:48,300
Think about it.

340
00:13:48,300 --> 00:13:50,534
This one's not really going to
get the right image anymore.

341
00:13:50,534 --> 00:13:51,600
It's going to get some of both.

342
00:13:51,600 --> 00:13:52,768
And this one's going to get some of both.

343
00:13:52,768 --> 00:13:53,683
It's going to be blurry.

344
00:13:53,683 --> 00:13:57,500
Your head would have to be
perfectly level the whole time,

345
00:13:57,500 --> 00:13:59,400
which might be annoying.

346
00:13:59,400 --> 00:14:01,768
So what we do is, instead,

347
00:14:01,768 --> 00:14:04,567
we create circular polarized glasses,

348
00:14:04,567 --> 00:14:07,613
so that this one would
only get one polarization,

349
00:14:07,613 --> 00:14:10,767
this one would get the other direction.

350
00:14:10,767 --> 00:14:14,367
This way, even if you tilt
your head a little bit ...

351
00:14:14,367 --> 00:14:16,033
shoot, clockwise is clockwise,

352
00:14:16,033 --> 00:14:18,200
counterclockwise is counterclockwise.

353
00:14:18,200 --> 00:14:21,869
By using circular
polarization for 3D movies,

354
00:14:21,869 --> 00:14:24,500
it can make it a little easier on you eyes

355
00:14:24,500 --> 00:14:26,567
to see a better 3D image,

356
00:14:26,567 --> 00:00:00,000
even if your head's tilted a little bit.