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PROFESSOR: In this
demonstration,

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we're going to study the
reflectivity of light

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at a dielectric interface
or a piece of glass.

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The setup is here.

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We have a laser, a
helium neon laser.

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He's the beam from the laser.

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It will be reflected
by a mirror,

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and then we pass it through
a quarter wave plate.

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So the light here is
circularly polarized,

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and then we pass the
circularly polarized light

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through this polarizer.

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So that by varying
the transmission

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axis of the polarizer, we can
vary the state of polarization

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of the light coming
through the polarizer,

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and the light goes onto
a piece of glass here.

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Then we have the
transmitted light,

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which is here, falling
on this circular screen,

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and the reflected light
falls on the screen also.

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So by varying the angle
of incidence here,

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we can vary the
spot on the screen.

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First, we'll start with
vertically polarized

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light or transverse electric
by setting the polarizer here

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along the vertical axis,
and the white arrow

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indicates vertical polarization.

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So we'll start with a small
angle of incidence close

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to zero, and then
we'll slowly work up.

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And what you want to
watch is, here, we

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go through 20 degrees,
30 degrees, or so on.

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What you see is that the
intensity of the light

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increases as we
approach 90 degrees.

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We're here, 80, and then 90.

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And you see that the
light gets pretty bright.

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Now we go back again.

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It is 50 degrees, 40 degrees,
and so on all the way down

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to zero angle of incidence.

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Now what I'm going to
do is change the state

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of polarization to
the horizontal one

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by changing the transmission
axis of the polarizer.

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And then now, let's
look at the reflectivity

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as a function of angle
of incidence here,

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again, with polarization along
the horizontal plane or p

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polarization.

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Let's see now what happens.

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So we start, again, about 4%.

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Here's 10 degree marker,
20 degrees, 30, 40.

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Now you can see that the light
is getting dimmer and dimmer.

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Here is 50, and here, we
reach the Brewster angle,

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where the intensity is zero.

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And then here is
360 degrees, 70, 80.

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And you can see the light
intensity is getting

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bigger, and bigger, and bigger.

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In order to look at it
a little bit better,

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let's dim the room lights
and do the experiments again.

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We're going to start with
vertical polarization,

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or s polarization, or if you
wish, transverse electric.

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And then we'll start varying the
angle of incidence from zero.

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There is 10 degrees,
20, and you can

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see that the light
intensity is increasing.

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Here's 30, 40, and you can
see it's increasing nicely.

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50 degrees, here is 60, 70.

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Now it's really
getting bright, 80.

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And then as we
approach 90, you can

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see that the angle,
the reflected beam,

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is more intense than
the transmitted beam.

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And here, we are
very close to 90.

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And then, again, you
see the other spots

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due to multiple reflections.

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Let's go back now.

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Here we are.

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The intensity is going down.

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Here's the 60 degree
marker, 50 degrees, 40, 30,

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and so on all the way to zero.

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Now what I'll do, I'm going
to change the polarization

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to horizontal polarization
or p polarization.

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But just to make
sure I have it right,

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I'm going to do a little
check on the Brewster angle,

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and then we'll start again.

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So here now with this
horizontal polarization,

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I'm going to now vary
the angle of incidence

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as we watch this
spot on the screen.

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You can see the spots.

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The intensity is going down.

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Here's the 20 degree
marker, 30, 40.

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And now, the spots are
getting dimmer, even more, 50.

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And as we get close
to the Brewster angle,

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it goes to zero at 56.5, then
here at 60 degrees, 70 degrees.

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Now it's picking up
intensity fast, 80.

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And as it gets close to 90
degrees, intensity heads

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towards 100%, and
suddenly, looks brighter

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than the transmit in intensity.

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Also, of course, you see the
effect of multiple reflections,

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so let's go back.

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It's 80, 70, 60,
getting nice and dim.

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And at 56.5, it goes to zero
and then picks up, again, at 50.

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So now, let's go back
to the Brewster angle,

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where it's extinguished.

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But the light is
pretty close to zero,

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and let me then change the
polarization back to vertical.

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Here we are.

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Go back to the vertical,
and we see that the spot

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gets brighter and brighter.

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So here, it is with
vertical polarization,

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and here is the intensity
with horizontal polarization.

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In summary, we have
shown how light

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is reflected at a dielectric
interface or a piece of glass.

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We've shown that
for polarization

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in the vertical plane
or s polarization,

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the reflectivity goes from about
4% for zero angle of incidence

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to 100% for 90 degree
angle of incidence.

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In contrast, for horizontal
polarization or p polarization,

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the reflectivity,
again, starts at 4%,

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but goes down to zero
at the Brewster angle.

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And in this case, for
glass is 56.5 degrees

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and then goes rapidly
up to 100% for 90 degree

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angle of incidence.