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PROFESSOR: Now,
we're ready to look

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at diffraction by thin fibers
or wires, opposite to slits.

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The setup is
essentially very simple.

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We even took out the
lens that we had here

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so the laser beam can go
directly onto these wires.

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So if we take a close
look at what we have here,

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we have just four
wires then I can just

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put in the way of
the laser beam,

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and then we, on the screen, can
see the associated diffraction

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pattern.

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So let's start with
the thickest wire.

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So you can see on
the screen, here

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is the diffraction pattern
associated with this wire.

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And what you can see--

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well, first of all,
let me remind you

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that we have the little
circles there, which

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are the 5 centimeter markers.

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So you get a feel
for the spacing.

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Again, the distance between
the wire and the screen

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is 200 centimeters, and
the wavelength of the laser

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is 6328 angstroms.

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I'm not going to
tell you the diameter

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of the wire, because
I'm going to let

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you work that out for yourself.

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But first you want to look
at the diffraction pattern.

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And you can see it looks very
similar to the single slit

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diffraction pattern, except
for the blob in the center.

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And the blob in the center
is a little complicated,

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but I can attenuate
the laser beam.

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And you can see that
it is pretty bright,

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because essentially
it is the laser beam.

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And so it's very difficult
to get information from it.

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The information is hidden
in the lobes on the side

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and the spacing between them.

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Because that will
be then related

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to the diameter of the wire.

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So then, for the
thickest wire, we

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see this kind of pattern,
this kind of spacing.

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Now, let me move on to wire
number two, which is thinner.

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Here's wire number two.

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And we can see
that, first of all,

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there's less light in the wings,
because the wire is thinner.

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So we don't see as
many side lobes.

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And then, again, you'll see
this ugly blob in the center.

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But the information is
in the lobes on the side.

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And you can see
them very clearly,

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and you can see their
spacing very clearly.

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Now, let me go on to
wire number three, which

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is, again, thinner still.

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And maybe here we can
zoom in a little bit

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so that we can see it
a little bit better.

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And again, you can see the
central spot is a little messy,

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but the information,
as I said before,

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is in the fringes on the side.

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Now, here, again, if I
reduce the laser intensity,

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you see what the central
fringe looks like.

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And as I bring it up, we'll
see the rest of the fringe.

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Now,

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Finally, I'm going
to go to my thinnest

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wire, which is over here.

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And now we may have to
zoom out a little bit

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so that we can see the spacing.

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00:04:06,438 --> 00:04:08,730
And maybe we can increase
the sensitivity a little bit,

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because this isn't
all that much light.

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So we can see good.

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Now, we can see the
separation between the fringes

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in the wings.

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So I hope you've been
watching carefully the spacing

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between the fringes,
so that you can come up

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with the diameter of
the individual wires.

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Now, this brings
me to the end of

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the one-dimensional Fraunhofer
diffraction patterns.

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Next, what we're
going to do is look

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at two-dimensional Fraunhofer
diffraction patterns.

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And I assure you,
they're even more fun

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than the one-dimensional.