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PROFESSOR: Now, we're going to see
what happens when the beam going into the cavity
is an expanding beam.
What we've done, we've added this lens to the setup
so that the beam leaving the lens is an expanding beam.
And I don't know whether you can see it that well on the card,
but the beam is an expanding beam.
It goes into the cavity, and they transmitted beam then
goes to the screen over there.
And if we can see it in close-up,
we can see that we have rings instead of the spot
that we had before.
And as the cavity length changes,
the diameter of the rings change.
As you can see in the insert.
Just as a special treat for you, we're
also going to look at the reflected light
from the cavity.
Again, we've added this beams splitter here
so we can sample the light.
And then, we'll go from the beam splitter, onto this mirror,
onto the screen.
Now, the spot on the left is associated
with the reflected beam, and the spot on the right
is that associated with the transmitted beam.
Now, you can see that they're rings.
Both of them are rings.
In the transmitted beam, the rings are bright.
In the reflected beam, the rings are dark.
Now, what I'm going to do is take off the scan
and do the scanning by hand.
Now, let's go back and look at the two sets of rings.
You can see, as I push a little bit, that the one on the right,
let's say, when it goes right, the one on the left is dark.
I can also do it by applying a voltage
to the piezoelectric crystal, again, by hand,
and vary it by hand.
And you can see here that, again, the one on the right,
I can make the central spot bright.
You can see the one on the left, or the reflected beam, is dark.
And when the transmitted beam is dark in the center,
the reflected beam is bright.
So indeed they are opposite one another.
Now, the interesting thing is, first of all, why
do we have rings?
And also, why the behavior is as shown?
And, again, this is left to you to think about.
In the next demonstration, instead
of using a plane parallel cavity, as we've used so far,
we're going to use a cavity with curved mirrors.
And I assure you it has lots of interesting things for us
to learn from.