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PROFESSOR: Now, we
are ready to look

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at Fraunhofer
diffraction associated

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with circular apertures.

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The setup, again, is
the same as before.

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But I just want to
remind you of it.

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We have a helium-neon laser.

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Beam from the laser is
reflected by this mirror

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and then reflected again by
this mirror into this lens.

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And now we have the
lens focusing the light

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onto a set of apertures.

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And then the diffraction
light from the aperture

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then goes onto the screen.

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Now, these apertures
have different diameters.

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Actually, they're tiny pinholes.

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So the first one we have
has a 100 micron diameter.

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So, now, if we
look at the screen,

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we see the diffraction pattern
associated with 100 micron

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diameter aperture.

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Again, let me give
you some dimensions.

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The separation between the
aperture and the screen

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is about 100
centimeters, the light

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is from the helium-neon
laser at 6328 angstroms,

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and the scale on the
screen from here to here,

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which is the diameter
of the first dark ring,

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is about 1.6 centimeters.

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So, now, you can check
the diffraction pattern,

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given the information that
I've just presented to you.

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00:02:01,770 --> 00:02:03,105
At the moment, you see--

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00:02:06,502 --> 00:02:08,169
first of all, let me
do some adjustment,

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make sure that I'm picked up.

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And what you see, you see
a bright ring in the middle

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and then some faint rings
around the central bright ring.

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There are actually many rings,
but because they're so faint,

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you can't see them.

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So what I'm going to do
is pull away this screen

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and expose to you a
more sensitive screen,

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as we can see.

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Now, you see that we're
saturating in the middle,

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but we're beginning
to see other rings.

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00:02:41,740 --> 00:02:44,890
Now, if we open up the
camera aperture some more,

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you can see even more rings.

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And, again, all these rings,
and their intensities,

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and sizes, and what
have you can be

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00:02:52,240 --> 00:02:56,770
calculated from the information
that I've given you.

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So, now, this is then the
Fraunhofer diffraction pattern

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associated with 100
micron aperture.

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Now, I would like to move
to the next aperture, which

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is 50 microns.

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And then I'm going to
do a little tweaking

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so I have it nicely picked up.

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Good.

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So, now, if we can look
at it on the whole screen,

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this is then the diffraction
pattern of a 50 micron

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aperture, and you can see that
the size of the central fringe

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is bigger.

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And, again, from the
information that I've given you,

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you should be able to
calculate what it should be.

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00:03:44,810 --> 00:03:49,700
Again, if we open up the
aperture in the camera,

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00:03:49,700 --> 00:03:55,090
maybe we can start to
see a few more rings.

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00:03:55,090 --> 00:03:57,200
And, again, the light
level is smaller

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than with the previous aperture,
because, again, the aperture

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is only 50 microns this time.

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Now, we've seen the Fraunhofer
diffraction pattern associated

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with a rectangular aperture.

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And then, just now, we saw
the diffraction associated

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with a circular aperture.

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In the next
demonstration, we're going

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to show the Fraunhofer
diffraction pattern associated

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with multiple slits-- with
a two-dimensional array

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of multiple slits.

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So when we come back,
we'll have the setup

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arranged so we can demonstrate
that effect for you.