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PROFESSOR: Now, I'm going to
show another way of reducing

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fringe contrast.

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This one has to do with the
polarization of the light

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coming out from each arm
of the interferometer.

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The setup is the same
as we had before,

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except that I've
added a polarizer here

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to clean up the
polarization before we

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enter the interferometer.

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So first, let's check on
the polarization coming out

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of each arm the interferometer
in the present setup.

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And I'll do this by putting
a polarizer out here.

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And then I'm going
to block each arm

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and check on the polarization.

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Now, as we can see
on the screen, when

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the arrow of the polarizer
or the transmission

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axis of the polarizer
is along the horizontal,

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I have a lot of light.

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And when the arrow is vertical,
I've extinguished the light.

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Which means that
the light coming out

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from this arm of
the interferometer

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is plane polarized in
the horizontal direction.

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That's for this arm.

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Now, I'll check on the beam
coming out from the other arm--

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from this arm here.

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And, again, there's
a lot of light

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when the arrow is horizontal,
and it's extinguished

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when the arrow is vertical.

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Which says that the polarization
of the light coming out

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of the two arms is the same
in the horizontal direction.

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Now, I'm going to rotate
the plane of polarization

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of the light coming
out from this arm

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by using this
quarter-wave plate, which

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I will insert in this arm.

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Now, as we know, light going
through a quarter-wave plate,

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twice its polarization will
be rotated, as we'll see.

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So now I want to show that
the light from this arm, going

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through this polarizer
here, onto the screen, it's

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going to have its
plane of polarization

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changed as I rotate
the alignment

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of this quarter-wave plate.

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So with the arrow over here, we
can see we have a lot of light.

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And as I rotate the
quarter-wave plate,

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I see that I can
extinguish the light.

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Which means now,
the polarization

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of the light coming
out from this arm,

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is orthogonal to
the transmission

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axis of the polarizer.

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So good.

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So, in this way, then
I can rotate the plane

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of polarization
from 0 to 90 degrees

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by simply rotating the
quarter-wave plate.

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Now, I'm ready to look at the
effect of polarization rotation

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on the fringe contrast.

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And what I'll do first, I
will take out the polarizer.

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So now I have, then,
two beams coming out

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from the interferometer.

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One has polarization in
the horizontal plane.

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This one here, I can change the
state of polarization anywhere

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from horizontal to vertical.

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So let's look at the screen.

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And we see that we have good
contrast in this position.

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Then, as I rotate the plane
of a polarization of this arm

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by 90 degrees, you can see that
the fringe contrast disappears.

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And, in fact, in this
position, you don't even

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see any fringes at all.

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If I go back to the
original position,

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you can see that the
contrast comes back.

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And we go back again,
watch how the fringes,

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because the polarizations
are orthogonal.

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And just to show you
that, indeed, I'm

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not hiding anything,
I'm going to show

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that there's still light
coming out from each arm--

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light coming out from
this arm and light

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coming off of this arm.

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But when I superimpose them,
there's no interference.

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Now, let me put it back
to the position where

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the polarizations are equal.

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And, indeed, we get the
good contrast back again.

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So, in conclusion, we've shown
that orthogonally polarized

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light does not
interfere, and that's

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why we get very low
contrast fringes when

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we try to interfere
orthogonally polarized light.

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So then, in any
interferometer experiment,

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you have to make sure
that the polarization is

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the same coming out from each
arm of the interferometer.

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The question I want
to leave with you

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is why orthogonally polarized
light does not interfere.