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PROFESSOR: Now, I'm
going to discuss

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another method of
reducing fringe contrast,

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and this one is associated
with the intensities in the two

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arms of the interferometer.

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Let's look at our present setup.

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And again, let's look
closely at the fringes.

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And we see the good contrast
that we've had before.

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Let's check on the
intensities in the two arms.

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So what I'm going to
do here, block one arm.

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You can see the
intensity of one arm.

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And then block the other
arm, and you can see

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the intensity of the other arm.

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Again, you can see that the two
intensities are pretty equal,

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and we get pretty good contrast.

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Now, I'm going to change the
relative intensities in the two

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arms by putting an attenuator.

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Here is an attenuator that
I'm placing in this arm,

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so that the total attenuation in
the beam coming out of this arm

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will be a factor of 10.

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Let's look at the intensity
coming from this arm.

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You can see, as before,
it's pretty bright.

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Let's look at he intensity
coming from the other arm,

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and it's attenuated
by a factor of 10.

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Let me remove the card, and
let's look at the interference

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pattern.

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And then you can see that
the interference pattern here

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is a loss in
contrast, especially

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in this area around here.

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The good contrast over
here can be explained,

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but I don't want to
go into it right now.

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But the one that
we're interested in

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is the bad contrast
around this region.

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Because if I take this out--

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take the attenuator out--

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good contrast comes back.

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If I put it in, I get that
poor contrast on the side.

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Now, to go further, I'm going
to put a bigger attenuator.

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This time, I'm going to
attenuate the beam coming out

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of this arm by a factor of 100.

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So again, let me show you the
intensity in the other arm.

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As it was, before pretty good.

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If I look at the
attenuated arm now,

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you can't even see
the beam at all.

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I've attenuated by
a factor of 100.

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Let me remove the card so I
can look at the interference

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pattern.

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And you can still make out a
little bit of interference.

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The contrast is very
poor, but at least you

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get a feel that there
is interference going on

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with full contrast, because the
two intensities are unequal.

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And again, if I take it out, you
can see the contrast is back.

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Put it back in again, and
the contrast is very poor.

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So, in conclusion,
if you want to get

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good contrast in the fringes--

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and that's where
the information is--

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you have to make sure that the
intensities in the two arms

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are equal.

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Otherwise, you have
poorer contrast.