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PROFESSOR: Now,
we're going to see

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what happens when the
beam going into the cavity

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is an expanding beam.

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What we've done, we've
added this lens to the setup

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so that the beam leaving the
lens is an expanding beam.

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And I don't know whether you can
see it that well on the card,

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but the beam is
an expanding beam.

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It goes into the cavity, and
they transmitted beam then

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goes to the screen over there.

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And if we can see
it in close-up,

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we can see that we have
rings instead of the spot

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that we had before.

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And as the cavity
length changes,

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the diameter of
the rings change.

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As you can see in the insert.

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Just as a special
treat for you, we're

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also going to look at
the reflected light

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from the cavity.

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Again, we've added this
beams splitter here

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so we can sample the light.

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And then, we'll go from the
beam splitter, onto this mirror,

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onto the screen.

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Now, the spot on the
left is associated

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with the reflected beam,
and the spot on the right

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is that associated with
the transmitted beam.

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Now, you can see
that they're rings.

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Both of them are rings.

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In the transmitted beam,
the rings are bright.

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In the reflected beam,
the rings are dark.

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Now, what I'm going to
do is take off the scan

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and do the scanning by hand.

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Now, let's go back and look
at the two sets of rings.

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You can see, as I push a little
bit, that the one on the right,

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let's say, when it goes right,
the one on the left is dark.

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I can also do it by
applying a voltage

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to the piezoelectric
crystal, again, by hand,

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and vary it by hand.

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And you can see here that,
again, the one on the right,

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I can make the
central spot bright.

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You can see the one on the left,
or the reflected beam, is dark.

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And when the transmitted
beam is dark in the center,

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the reflected beam is bright.

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00:02:54,500 --> 00:02:58,850
So indeed they are
opposite one another.

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00:02:58,850 --> 00:03:02,980
Now, the interesting thing
is, first of all, why

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do we have rings?

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00:03:05,380 --> 00:03:10,870
And also, why the
behavior is as shown?

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And, again, this is left
to you to think about.

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00:03:14,770 --> 00:03:18,400
In the next
demonstration, instead

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of using a plane parallel
cavity, as we've used so far,

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we're going to use a
cavity with curved mirrors.

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And I assure you it has lots
of interesting things for us

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to learn from.