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PROFESSOR: In this
demonstration,

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I'm going to illustrate one
or two interesting properties

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relating to the propagation
of light in a dielectric.

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The setup is very simple.

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Here we have a plane polarized
laser, helium-neon laser.

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Here's the beam from the laser.

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We reflect it by this mirror
into a quarter-wave plate.

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Now, over here, the light
is circularly polarized.

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Then we pass the
circularly polarized light

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through a polarizer
over here, so that,

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as we rotate the transmission
axis of the polarizer,

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we can generate linearly
polarized light over here,

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with different planes
of polarization.

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For example, when the
arrow or the transmission

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axis or the polarizer
is horizontal,

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we have horizontally polarized
light, plane polarized light.

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And when the arrow is
vertical, for example,

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we have plane polarized
light in the vertical plane.

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So this polarized light enters
this dielectric cylinder here.

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Normally, one would
use a piece of glass.

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But in this case, I'm
using a piece of lucite,

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because I have a piece
of lucite available.

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And then the light,
after passing

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through the lucite cylinder,
goes off onto the wall.

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So, now, what I'm
going to do is change

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the polarization of light and
look at the light scattered

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from the rod.

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First of all, if we take a close
look at the rod, we can see--

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in fact, we can see very little.

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We can see a hotspot over here.

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That's the reflection
at this boundary--

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Fresnel reflection.

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And we also see a
hotspot over here,

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which is the reflection at
the exit face of the rod.

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We don't see any light--

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any beams of light--

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leaving the cylinder
at any other angle.

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However, what we do see--

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and I hope you can see it--

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is a faint streak
marking the direction

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of the laser beam in the rod.

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This is due to Rayleigh
scattering in the material.

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Now, let's look at
the polarization

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of the Rayleigh scattered
light in the material.

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Now, in the insert, you see that
the polarization of the light

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is vertical, and the
camera is looking

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in a horizontal direction
at the scattered light.

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Here you see-- you make out
there is some red light.

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Now, when I go to
horizontal polarization,

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see that the scattered
light is extinguished.

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When I go back to the vertical
polarization, linearly

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polarized light in
the vertical plane,

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you see that the light comes
back, and you can see it.

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But I know that this
is not a great display,

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so what we'll do now,
we'll turn down the lights

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so we can see the
effect much better.

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Now that the room
lights are dimmed,

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we can see the
effect much better

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in the lower part of the screen.

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In the upper right-hand corner,
you see the polarizer being

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rotated--

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or the transmission of
axis of the polarizer being

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rotated-- so that we can select
any plane of polarization

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we want.

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In this case, with
a vertical arrow,

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we have vertically
polarized light

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going into the lucite rod.

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Then you see that the scattered
light is pretty bright.

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Now, watch carefully.

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As I rotate the plane of
polarization of the light,

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of the incident light, you can
see that the scattered light

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is getting extinguished.

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In fact, when I have
plane polarized light

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in the horizontal plane, I see
very little scattered light.

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Let's do it again.

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Let's go back to
the plane polarized

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light in the vertical plane.

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And you see a lot
of scattered light.

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And where the horizontal
plane, when incident light

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is polarized in the
horizontal plane,

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you see very little
scattered light.

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Remember, the camera is looking
from this side of the lucite

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too.

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In summary, we've seen one
or two interesting properties

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relating to the
propagation of light

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in a dielectric material
like a piece of lucite.

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We saw that the scattered light
from the beam within the rod

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varies with the
incident polarization.

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And we're leaving
it to you to explain

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that when the incident
polarization is plane polarized

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in the vertical plane, we
saw a lot of scattered light

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when we looked from the side.

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And when the
incident polarization

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was plane polarized in
the horizontal plane,

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we saw almost no
light at all scattered

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from the beam within the rod.

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So this is left
to you to explain.