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PROFESSOR: In this
demonstration,

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we're going to study the
interference of light.

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In particular, the interference
of two beams of light.

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And we will use a simple
Michelson interferometer

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to study the interference.

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And here it is.

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Here we have a
helium neon laser,

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and here is the beam from
the laser being reflected

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by this mirror here, and then
another mirror over here,

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into the Michelson
interferometer.

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We have a 50-50 beam splitter,
which reflects half the light

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into this mirror in this arm.

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Then the mirror
reflects the light back

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through the beam splitter,
again reflected by this mirror

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here into this lens, and
then onto the screen.

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The second arm,
we have the light

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transmitted through
the beam splitter

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onto the mirror
in the second arm.

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The mirror reflects light back
to the beams splitter, which

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is then reflected here onto
this mirror, through the lens,

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and onto the screen.

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So now I'm going to cover
up one of the mirrors

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or block one arm of
the interferometer.

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And then, let's look
at the screen now.

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And we'll see only the
light from this arm.

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Now, if I block the arm
here and take the card away

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from this arm, now
you'll see the beam

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from the arm over here.

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So in order to see
interference, I'm

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going to lift this card so that
both beams are superimposed.

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And then, as we
see on the screen,

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we see the fringes as a result
of the interference of the two

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beams, one coming from this arm
and one coming from this arm.

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You see that there is
a very nice contrast

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between the bright and the
dark parts of the fringe.

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And the contrast is
where the information is.

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If you have no contrast at
all, then essentially you

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have no no interference.

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Now, I want to illustrate how
sensitive this interferometer

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is.

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All I have to do is
lean on the table here,

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and you can see I can
make the fringes move

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due to a misalignment
of the interferometer.

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Another way is by actually
misaligning the interferometer

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by adjusting the alignment
of this mirror here.

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You can see, every time
I touch the mirror,

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there's shaking, because
I'm disturbing the setup.

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But if I take my hand away,
the stability is restored.

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So again, I'm going to
misalign the interferometer.

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And you can see that
I can vary the spacing

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between the fringes, which
is due to the misalignment.

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Also, you can see that if
I can work hard enough,

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I can make the field
almost dark by making

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the spacing between the
fringes so large that--

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here we are.

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You can see very nicely
how it's going in and out,

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and I can press on the
table to make the light go

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completely out and onto
maximum value simply

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by leaning on the table.

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This means that the beams
are practically superimposed

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almost exactly on each other.

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Let me go again back to where I
was with the original alignment

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and show you another way of
altering the path difference.

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In this way, I want to
do it in a controlled way

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by mounting this mirror here.

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If we can get a
close-up at this mirror,

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we can see that this
mirror is mounted

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on a piezoelectric
crystal over here.

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And when I apply a voltage
to the piezoelectric crystal,

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I can make the
length of it change.

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Which means that this length
of this arm will change,

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and then we'll see
the fringes move.

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So let's do it.

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Now, I'll connect
my voltage source

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to the piezoelectric crystal.

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And then, now, if we
can see the fringes,

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you can see they're
pretty stable.

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And if I turn my
voltage source, you

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can see that the fringes
go backwards and forwards,

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because I'm applying a
slowly varying sinusoidal

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voltage onto the
piezoelectric crystal, which,

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in turn, is modulating
the length of the arm

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of this interferometer.

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So, in summary, we've seen how
two beams of light interfere.

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We looked at the
interference pattern.

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We looked at the
contrast and the fringes.

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We looked at the spacing between
fringes due to misalignment.

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And we also looked
at how sensitive

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the setup is to very
small perturbations

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of the order of wavelengths of
light to the interferometer.