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SHAOUL EZEKIEL: In
this demonstration,

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we're going to show a
simple way of rotating

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the plane of polarization of
laser light from zero degrees

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to anywhere to 90 degrees using
simple techniques, for example,

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by the use of polarizers.

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The setup that we're
going to use is here.

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We have a laser,
helium neon laser.

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And here is the beam
of the laser light.

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We're going to reflect
it off this mirror

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and then reflect it again
off this second mirror.

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And then we'll go
through a polarizer.

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And then the light coming
out of the polarizer

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will go onto the screen.

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The purpose of this
polarizer is to establish

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the state of polarization
of the light coming out

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of the polarizer.

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If the polarizer is
any good, the state

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of polarization
after the polarizer

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should be plane polarized.

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Now the little arrow here
labels the transmission axis

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of the polarizer, which means
that the state of polarization

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of the light then is
aligned with that arrow.

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Now let us show, indeed, that
the state of polarization

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of that polarizer is
indeed as we said it was.

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So we take another polarizer.

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And we put it over here.

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Again, the arrow labels
the transmission axis

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of this second polarizer.

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Now, when I have
the arrow vertical,

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the same as in this case, the
light is around its peak value.

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It's very difficult
for us to see

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what's happening
around the peak value.

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So it's nice to turn
the transmission

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axis of this polarizer until
we extinguish the light, which

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means that the transmission
axis of this polarizer,

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which is going to be
referred to as the analyzer,

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is orthogonal to the axis here.

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And the fact, as we look
on the screen, the fact

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that we can null the light
means that the polarization

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here before entering
this analyzer

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is, indeed, plane polarized.

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And the direction
of the arrow here,

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which labels the
null intensity, which

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means that the transmission
axis now of the polarizer

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is orthogonal to the
polarization of the light,

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is then indicated by the
direction of the arrow.

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But we all know that the true
polarization of the light

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is in the vertical direction.

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But it's much easier
for us to label

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the block axis, the
direction of the block axis.

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So please keep in mind
that even though I

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might show you the
null on the screen,

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the actual polarization is
90 degrees away from that.

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Now I'm going to
demonstrate the rotation

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of the plane of polarization
by means of the polarizer.

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First, what I'm going to do
is set this analyzer back

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to where it was, set
the arrow vertical,

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and then take another
polarizer, then,

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to rotate the plane
of polarization.

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Now, at the beginning, I'm
going to set this polarizer

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arrow, which again
labels the transmission

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axis of this polarizer, in
line with all the others.

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Then I want to rotate the plane
of polarization of the light

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by this polarizer,
let's say by 45 degrees.

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Well, in order to
check that, indeed, I

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have rotated by 45
degrees, again I'm

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going to track the null.

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So now if we look on the screen,
and I rotate the analyzer,

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let's see where
null is this time.

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And here it is.

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We can see that null
is when the arrow is

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in this direction, which
is, again, 90 degrees

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to the position in
this direction, which

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is about 50 degrees or so
from where it was before.

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Now let me go back to
aligning the analyzer

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with the peak transmission.

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And you can see that
the peak transmission--

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it's not that difficult to see.

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But you can see that
it is about half or so,

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about half the intensity of
the peak transmission that I

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had when this polarizer was
in the vertical direction.

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Now, if I want to rotate
the polarization some more,

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let's say close to
90 degrees, again

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track the direction of the null,
now, as we watch on the screen

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here, the position of
null now is over here.

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And it shows that the arrow is
pretty close to vertical, which

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means, again, we've rotated
the plane of polarization

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some more.

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Now, the interesting thing
about going close to 90 degrees

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is that the peak transmission,
the peak transmission

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of the light, as you can see
over here, is not so big.

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And, in fact, if I go
even closer to 90 degrees,

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I don't even get any
light transmitted

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through the polarizer.

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So the conclusion,
then, is that I

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can rotate the plane of
polarization of light

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using a simple polarizer.

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But the penalty is that
as I get close to 90

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degrees of
polarization rotation,

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I get very little
light transmitted.

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So in order to
overcome this problem,

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we use a second polarizer.

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And we place the
second polarizer.

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In this case, I'll place
it before the first,

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before this polarizer here.

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And remember, even
before I put it in,

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that the polarization
of the light

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now is along this axis, which
is 90 degrees with respect

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to this one.

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And that's why we didn't get
any light through the analyzer.

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Now, I'm going to put
this polarizer here.

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And this polarizer is still
along this direction, which

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is orthogonal to this one.

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And before, remember, there
was no light coming through.

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And now you can see by adjusting
the axis of this polarizer

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here, I can get some
light to come through

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with polarization that is
orthogonal to the original one.

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And again, to go demonstrate
that the null, now,

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is along the vertical
direction, which

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means I have achieved
90 degree rotation using

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now two polarizers and
still have light left.

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The sad story is
that you only get

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one quarter of the light
coming out when you rotate

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the polarization by 90 degrees.

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But a quarter of the light
at 90 degree rotation

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is better than no light at all.

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So to summarize, if you
use only one polarizer,

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you can rotate the
state of polarization

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by, let's say up
to about 45 degrees

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or so and only lose
half the intensity.

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If you want to go more, then
you pay a penalty in intensity.

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If you use two polarizers,
then you can rotate any way

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you want.

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And the worst thing you can
do, at 90 degree rotation,

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is that you get only a quarter
of the light coming through.