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PROFESSOR: In a
previous demo, we

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studied how light is
reflected and transmitted

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at a dielectric interface
or a piece of glass.

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In that demo, we studied
the air-glass interface.

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In this demo, we're going to
do it the other way around.

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We're going to look at
reflection and transmission

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of light at, again,
a glass interface.

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But we're going to come
in from the other side

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and look at the
glass-air interface.

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The setup is here.

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We have a helium-neon laser.

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Here's the beam from the laser.

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We're going to reflect
it by this mirror

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and passed it through
a quarter-wave plate.

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Now, the purpose of this
quarter-wave plate is to make

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the light in this region
circularly polarized,

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so that when we pass
it through a polarizer,

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the light out here can be
changed in polarization

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by simply rotating
the polarizer here--

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by rotating the transmission
axis of the polarizer.

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And the white arrow indicates
the transmission axis

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of the polarizer.

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We could have also done
this using a polarizer

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and a half-wave plate.

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But then, when you rotate
the half-wave plate,

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the polarization changes
by twice the angle.

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So for you, the viewers,
you will get confused.

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00:01:58,240 --> 00:02:02,010
Here, it's simpler because
we you can see that indeed we

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can rotate the
polarization by 90 degrees

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just by watching this arrow.

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And in addition,
the intensity here

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will also remain constant,
even though the polarization

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is being rotated by 90 degrees.

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Now, this plane-polarized beam
now impinges at the interface.

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Now in this case,
we have a prism.

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And the light coming
into the prism

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will enter at this
surface and then impinges

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at this interface.

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And this is our
glass-air boundary.

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And then, the transmitted
beam will come out

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in this direction.

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And the reflected beam will
come out in this direction.

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Let me illustrate this.

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Here's the transmitted beam
that hits the circular screen.

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And here is the reflected
beam for a certain angle

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of incidence.

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And when I mention
angle of incidence,

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I mean the angle of
incidence with respect

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to the glass-air interface.

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And as I vary the
angle of incidence,

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you can see that the transmitted
beam will vary in angle,

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and the reflected beam
will vary in angle.

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The interesting thing
about the transmitted beam

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is that as I increase
the angle of incidence

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to about 41 degrees--

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if you watch the
spot on the screen,

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as I bring it to 41 degrees--

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the spot disappears.

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That means I've reached
the critical angle.

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And at that angle, we get
total internal reflection.

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And the transmitted
beam gets extinguished.

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And all the light comes out
along the reflected beam.

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What's interesting about
total internal reflection,

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or just before that, is
that the beam itself--

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let's go back just a little
bit before total internal

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reflection-- the
beam comes out right

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along the surface of the glass.

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00:04:15,480 --> 00:04:19,089
And in order to demonstrate
this a little bit better,

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I'm going to set up a little
better way of observing this.

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And I'm going to place
this screen on this rail,

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so that I can move it
right along the rail.

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So you can observe
where the beam comes out

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from the glass,
which is about here.

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And then, as I move
it out, you can

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see that the beam,
the transmitted beam,

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is coming out at an angle,
as you observe it move along

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the surface of the card.

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00:04:57,540 --> 00:05:01,660
So this is then the transmitted
beam at an angle of incidence

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that is smaller than
the critical angle.

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As I approach the critical
angle, the beam should--

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whoop, too much-- the beam
should travel pretty much

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00:05:15,700 --> 00:05:18,800
along the surface of the glass.

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00:05:18,800 --> 00:05:25,030
So now, as you can see,
I have the card right

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where the spot is on the glass.

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And now, I'm going to move it
along the surface of the glass.

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And you can see that
this spot essentially

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hugs the surface of the glass.

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00:05:35,750 --> 00:05:37,030
Let's do it again.

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00:05:41,060 --> 00:05:43,460
And here, you can
see that the spot

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hugs the surface of the glass
and goes on to the screen.

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Now, we're going to look
at the reflected beam which

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is this one over here.

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Since the intensity
of the reflected beam

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00:06:00,500 --> 00:06:06,110
varies as a function of angle
of incidence, what we'll do,

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we'll dim the room
lights a little bit,

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so that we can get a better
look at the variation intensity

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of the reflected beam.

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00:06:14,970 --> 00:06:17,390
Now that the room
lights are dim,

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and as we make a few camera
adjustments to enhance

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the effect, we're
ready to observe

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the intensity of
the reflected beam

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00:06:29,540 --> 00:06:32,870
as a function of
angle of incidence.

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Let me remind you, here
is the transmitted beam.

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And here is the reflected beam.

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00:06:41,270 --> 00:06:49,550
So let's go to zero angle of
incidence around about here.

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Now, let me remind you
that the polarization is

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in the vertical plane
or S polarization.

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Let's look at then
the reflected beam

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00:07:02,570 --> 00:07:07,070
as I increase the
angle of incidence.

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And what you see that the
intensity increases slowly

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and then, a little bit
faster here until I

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reach the critical angle.

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And around the critical
angle, you also

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observe that the
transmitted beam is just

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about ready to be extinguished,
or it is extinguished.

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And all the intensity is in
the reflected beam and stays

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high, supposedly
100%, all the way

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to an angle of
incidence of 90 degrees.

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Now, we go back, back,
and the intensity

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stays constant until we
reach the critical angle.

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00:08:04,292 --> 00:08:05,750
And from here, the
intensity starts

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to go down all the way to 4%
at zero angle of incidence.

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00:08:18,910 --> 00:08:21,310
So that was for S polarization.

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Now, I'm going to change the
polarization P polarization,

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or polarization in
the horizontal plane.

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Let's go and look
at the intensity

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of the reflected beam, starting
from zero angle of incidence.

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And as I increase the
angle of incidence,

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you can see now that the
intensity is starting to drop,

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while for S polarization
is starting to increase.

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In here, again,
for P polarization,

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the intensity drops
until an angle

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of 33.5 degrees,
which is the Brewster

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angle for glass-air interface.

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The intensity goes to zero.

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And then, it starts
to pick up again

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for angles bigger than 33.5.

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And you can see, it picks
up intensity very fast.

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And as we get close
to the critical angle,

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intensity is pretty high.

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And then, at the
critical angle, you

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can see the spot on the
left has been extinguished.

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That's the transmitted
beam being extinguished.

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All the light is in
the reflected beam,

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and it stays like that until
we reach an angle of incidence

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of 90 degrees.

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Let me go back now an angle.

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The intensity stays
constant until we

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reach the critical angle.

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Then, intensity starts
to drop, and drops

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to zero at the Brewster
angle, and picks up again

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on the other side.

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Now let me go back to
the Brewster angle.

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It's zero.

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And let me now change the
polarization to S polarization

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and show you that intensity
picks up for S polarization.

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And now, I will go
back to P polarization

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to extinguish this spot.

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In summary then, we have shown
how the intensity of light

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varies at the glass-air
interface, the reflected beam,

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and also for the
transmitted beam.

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00:10:44,960 --> 00:10:50,840
We have shown how we have
a Brewster angle for P

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polarization at 33.5 degrees.

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And we have also shown that
for the transmitted beam,

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at the critical angle,
the beam runs right

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along the surface of the glass.

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00:11:05,540 --> 00:11:09,770
Now, we're going to
look at the polarization

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or the state of polarization
of the reflected beam

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and also the transmitted
beam as a function

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00:11:16,520 --> 00:11:21,170
of the state of polarization
of the incident beam.

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00:11:21,170 --> 00:11:25,100
But before we do this, we
have to make a few adjustments

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00:11:25,100 --> 00:11:26,500
to our setup.