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PROFESSOR: Now, I'd
like to demonstrate

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multi-slit diffraction patterns.

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Now, normally one would
take a, let's say,

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piece of glass with
a ruling on it--

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many, many lines, many dark
lines on a piece of glass--

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and then one would shine
the laser light through it,

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and look on the screen, and
see the multi-slit diffraction

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pattern.

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But we're going to do
it a little differently.

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What we're going
to do, we're going

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to introduce one slit at a
time until we've build up

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to the many slits,
so you can see

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the contribution of each slit.

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And we're going to
do it in this way.

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We have the same
setup as before.

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Here's our laser,
and the two mirrors,

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and the lens to expand the beam.

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Here's the expanded beam.

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Now, over here, we
have two things.

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First of all, we
have a pair of jaws

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here, which are two
razor blades, which

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I can adjust the spacing
between the razor blades.

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I can adjust with this
translation stage over here.

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And then, right behind
the razor blades,

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there is a piece of glass,
which is a Ronchi ruling--

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just a piece of glass
with black lines.

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The thickness of each black
line is about 75 microns,

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and the spacing
between the black lines

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is about 125 microns.

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It's about 250 lines
per inch, if you want.

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So this is then
this Ronchi ruling,

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this piece of glass with all
these narrow slits on it.

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The screen is, as before,
200 centimeters away.

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And, again, the wavelength
of light is 6,328 angstroms.

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So now, let's look at-- first of
all, let's close down the jaws,

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look at the screen,
and see if we

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can see the single-slit
diffraction pattern.

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Here we have the single-slit
diffraction pattern.

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I have to apologize, because
when we close down the jaws,

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we don't have much light.

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But I hope you can still see
the single-slit diffraction

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pattern.

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Now, on the screen
we have the circles.

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The circles are the
5 centimeter markers.

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And the little arrow
tips, as you can see,

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they mark the 0s
of the central lobe

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of the single-slit
diffraction pattern

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associated with the 50 micron or
so slit on this Ronchi ruling.

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So now what I'm going
to do, I'm going now

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to separate the jaws
to admit one more--

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the second slit.

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Now, those of you who
understand this theory

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will quickly verify
that, indeed, you'll

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see three lobes then within
the single-slit principle lobe.

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Now, I'm going to add one
more slit by again opening up

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the jaws.

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As I bring in one more slit,
now we have three slits.

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And look at the pattern now.

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It generates some
weaker lobes in between.

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And the lobes themselves
are getting narrower.

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Now, we'll add one more.

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Here it is four.

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And here it is five,
six, seven, and so on.

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I'm just going to keep
enlarging the spacing,

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and you can see that, first of
all, the principle three lobes

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get narrower and narrower.

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And then you get a lot more
little side lobes in between.

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So as I widen the
spacing between the jaws,

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you can see that
those three lobes

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will get narrower and narrower.

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And, of course, you'll see
the ones from these other side

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lobes also.

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There's little dots to the side.

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Now, the intensity is
so bright that it's

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saturating our camera.

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So what we'd like to do,
we take a little close up

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so we can resolve the lobe.

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So if you can go in and take a
close look at the three lobes

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in the center, here we are.

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Then we go back again to when
I had only two slits in there.

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Now, I've added a third, a
fourth, a fifth, and so on.

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As you can see, the width
of the three principal lobes

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are getting narrower
and narrower.

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And now I have
lots of slits now.

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And, again, the
intensity is high,

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so I can't really
tell how narrow,

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but I know this
looks very narrow.

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So maybe we can cut down
the sensitivity a little bit

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on the camera.

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And let's see if we get a feel
of how narrow these spots will

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be.

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Again, all I can say,
they look pretty narrow.

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And I'll leave it
to you to calculate

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how narrow they become.

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Because I've given
you all the data.

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I've given you the
spacing between the slits,

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I've given you the
width of the slits,

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and the wavelength of the
light, and the distance

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between the slits
and the screen.

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So, in summary, this is a
very, very cute experiment

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demonstration of how the
addition of each slit

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contributes to the Fraunhofer
diffraction pattern.

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Now, we're going to look
at multi-slit diffraction

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as a function of line spacing.

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What we have here
are Ronchi rulings,

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which are pieces of glass with
lots of lines drawn on them.

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This one here has about
100 lines per inch.

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So we're going to put this
Ronchi ruling in here,

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in our setup.

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And the setup is the same
as before, with this lens

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here to enlarge the beam so
we can illuminate as many

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of the lines as possible.

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We've also added this
attenuator here so

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that we can adjust the intensity
of the light when we need to.

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So now let's look
at screen and see

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what we can see with this Ronchi
ruling of 100 lines per inch.

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As you can see, there are
plenty of very narrow dots.

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And, in fact, if you want to
get a feel for this scale,

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the little circles just
below the diffraction pattern

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are the 5 centimeter markers
that we've had before.

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Now, if I attenuate the
intensity a little bit,

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you can see that the
ones in the center

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are the brightest, of course.

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And, also, as I
reduce intensity,

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you can see that the spots
are really very small.

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Now, if I've given you the
number of lines per inch,

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and the spacing between
the Ronchi ruling

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and the screen is,
again, 200 centimeters,

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and the wave length
is 6328 angstroms,

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you should be able to check on
the spacing between the fringes

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and also on their widths.

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So here they are when
I overexpose them.

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So we can see the ones
way out in the wings.

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So this is then the
diffraction pattern--

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the Fraunhofer
diffraction pattern--

00:09:18.070 --> 00:09:22.160
associated with a Ronchi
ruling of 100 lines per inch.

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Now, let's look at
200 lines per inch.

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So here is the 200
lines per inch.

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And you can see that the
spacing now is different.

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But I leave it to
you to check on it.

00:09:38.310 --> 00:09:43.690
And, again, if I
reduce the intensity,

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and you get at least a little
bit of a feel for how narrow

00:09:47.380 --> 00:09:49.838
these dots are.

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They're indeed very
bright, because they're

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saturating our camera.

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So that's for then
200 lines per inch.

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Now we go to 300 lines per inch.

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Again, the spacing is different.

00:10:11.610 --> 00:10:16.420
And, also, if I change the
orientation of the lines,

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you can see that the diffraction
pattern also changes.

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So that's then for
300 lines per inch.

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The next one is
2,000 lines per inch.

00:10:31.110 --> 00:10:33.780
Now, when I put
it over here, you

00:10:33.780 --> 00:10:38.760
can see that the spacing
between the fringes

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are about 10 centimeters.

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And, again, that
gives you a check

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on the number of lines per
centimeter or per inch,

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as we have it.

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Now, let me see.

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If we pull back a little bit--

00:10:53.310 --> 00:10:58.590
pull back on the camera--
to see the other dots.

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Yes, here they are.

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But they're so widely
space, that it's

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difficult to get them all
on the camera at once.

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So if we go back to
the original position.

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If we go in again.

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Here we are.

00:11:12.930 --> 00:11:16.120
And now I'm going to
again reduce intensity.

00:11:16.120 --> 00:11:21.060
You can see how
narrow the spots are.

00:11:21.060 --> 00:11:27.120
So this then sums up
multi-slit diffraction pattern

00:11:27.120 --> 00:11:31.290
as a function of line spacing.

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In the next
demonstration, we're going

00:11:34.410 --> 00:11:37.710
to show the opposite effect.

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Instead of slits, we're
going to use thin wires.

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And then when we
come back, we'll

00:11:45.060 --> 00:11:47.400
show you what the Fraunhofter
diffraction pattern

00:11:47.400 --> 00:11:51.350
for very thin wires looks like.