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PROFESSOR: Now, I'd
like to demonstrate

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multi-slit diffraction patterns.

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00:00:26,220 --> 00:00:30,330
Now, normally one would
take a, let's say,

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00:00:30,330 --> 00:00:34,470
piece of glass with
a ruling on it--

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00:00:34,470 --> 00:00:39,473
many, many lines, many dark
lines on a piece of glass--

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00:00:39,473 --> 00:00:41,640
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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00:00:45,783 --> 00:00:47,700
But we're going to do
it a little differently.

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00:00:47,700 --> 00:00:49,560
What we're going
to do, we're going

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00:00:49,560 --> 00:00:54,150
to introduce one slit at a
time until we've build up

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00:00:54,150 --> 00:00:56,400
to the many slits,
so you can see

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00:00:56,400 --> 00:00:58,608
the contribution of each slit.

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00:00:58,608 --> 00:01:00,150
And we're going to
do it in this way.

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00:01:00,150 --> 00:01:02,620
We have the same
setup as before.

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00:01:02,620 --> 00:01:04,980
Here's our laser,
and the two mirrors,

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00:01:04,980 --> 00:01:07,020
and the lens to expand the beam.

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00:01:07,020 --> 00:01:09,580
Here's the expanded beam.

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00:01:09,580 --> 00:01:16,230
Now, over here, we
have two things.

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00:01:16,230 --> 00:01:18,780
First of all, we
have a pair of jaws

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00:01:18,780 --> 00:01:21,330
here, which are two
razor blades, which

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I can adjust the spacing
between the razor blades.

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00:01:24,330 --> 00:01:29,280
I can adjust with this
translation stage over here.

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00:01:29,280 --> 00:01:33,750
And then, right behind
the razor blades,

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00:01:33,750 --> 00:01:38,910
there is a piece of glass,
which is a Ronchi ruling--

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00:01:38,910 --> 00:01:41,550
just a piece of glass
with black lines.

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00:01:41,550 --> 00:01:48,900
The thickness of each black
line is about 75 microns,

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00:01:48,900 --> 00:01:52,950
and the spacing
between the black lines

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00:01:52,950 --> 00:01:56,595
is about 125 microns.

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00:01:59,610 --> 00:02:03,850
It's about 250 lines
per inch, if you want.

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00:02:03,850 --> 00:02:06,750
So this is then
this Ronchi ruling,

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00:02:06,750 --> 00:02:12,900
this piece of glass with all
these narrow slits on it.

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00:02:12,900 --> 00:02:17,980
The screen is, as before,
200 centimeters away.

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00:02:17,980 --> 00:02:23,880
And, again, the wavelength
of light is 6,328 angstroms.

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00:02:23,880 --> 00:02:28,920
So now, let's look at-- first of
all, let's close down the jaws,

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00:02:28,920 --> 00:02:33,180
look at the screen,
and see if we

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00:02:33,180 --> 00:02:36,780
can see the single-slit
diffraction pattern.

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00:02:36,780 --> 00:02:40,110
Here we have the single-slit
diffraction pattern.

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00:02:40,110 --> 00:02:43,560
I have to apologize, because
when we close down the jaws,

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00:02:43,560 --> 00:02:46,180
we don't have much light.

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00:02:46,180 --> 00:02:48,640
But I hope you can still see
the single-slit diffraction

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00:02:48,640 --> 00:02:49,140
pattern.

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00:02:49,140 --> 00:02:52,560
Now, on the screen
we have the circles.

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00:02:52,560 --> 00:02:55,080
The circles are the
5 centimeter markers.

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00:02:55,080 --> 00:03:00,600
And the little arrow
tips, as you can see,

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00:03:00,600 --> 00:03:04,950
they mark the 0s
of the central lobe

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00:03:04,950 --> 00:03:06,570
of the single-slit
diffraction pattern

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00:03:06,570 --> 00:03:15,870
associated with the 50 micron or
so slit on this Ronchi ruling.

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00:03:15,870 --> 00:03:18,120
So now what I'm going
to do, I'm going now

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00:03:18,120 --> 00:03:24,450
to separate the jaws
to admit one more--

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00:03:24,450 --> 00:03:26,340
the second slit.

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00:03:26,340 --> 00:03:31,110
Now, those of you who
understand this theory

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00:03:31,110 --> 00:03:33,660
will quickly verify
that, indeed, you'll

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00:03:33,660 --> 00:03:42,900
see three lobes then within
the single-slit principle lobe.

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00:03:42,900 --> 00:03:47,310
Now, I'm going to add one
more slit by again opening up

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00:03:47,310 --> 00:03:48,390
the jaws.

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00:03:48,390 --> 00:03:51,270
As I bring in one more slit,
now we have three slits.

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00:03:51,270 --> 00:03:52,890
And look at the pattern now.

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00:03:52,890 --> 00:03:55,860
It generates some
weaker lobes in between.

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00:03:55,860 --> 00:04:00,480
And the lobes themselves
are getting narrower.

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00:04:00,480 --> 00:04:02,640
Now, we'll add one more.

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00:04:02,640 --> 00:04:05,190
Here it is four.

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00:04:05,190 --> 00:04:13,480
And here it is five,
six, seven, and so on.

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00:04:13,480 --> 00:04:16,750
I'm just going to keep
enlarging the spacing,

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00:04:16,750 --> 00:04:21,459
and you can see that, first of
all, the principle three lobes

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00:04:21,459 --> 00:04:25,360
get narrower and narrower.

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00:04:25,360 --> 00:04:29,570
And then you get a lot more
little side lobes in between.

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00:04:29,570 --> 00:04:34,150
So as I widen the
spacing between the jaws,

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00:04:34,150 --> 00:04:37,990
you can see that
those three lobes

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00:04:37,990 --> 00:04:39,520
will get narrower and narrower.

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00:04:39,520 --> 00:04:43,300
And, of course, you'll see
the ones from these other side

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00:04:43,300 --> 00:04:44,050
lobes also.

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00:04:44,050 --> 00:04:47,840
There's little dots to the side.

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00:04:47,840 --> 00:04:51,010
Now, the intensity is
so bright that it's

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00:04:51,010 --> 00:04:52,930
saturating our camera.

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00:04:52,930 --> 00:04:56,590
So what we'd like to do,
we take a little close up

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00:04:56,590 --> 00:04:59,440
so we can resolve the lobe.

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00:04:59,440 --> 00:05:07,270
So if you can go in and take a
close look at the three lobes

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00:05:07,270 --> 00:05:19,015
in the center, here we are.

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00:05:19,015 --> 00:05:26,210
Then we go back again to when
I had only two slits in there.

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00:05:26,210 --> 00:05:31,110
Now, I've added a third, a
fourth, a fifth, and so on.

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00:05:31,110 --> 00:05:35,608
As you can see, the width
of the three principal lobes

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00:05:35,608 --> 00:05:37,025
are getting narrower
and narrower.

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00:05:42,900 --> 00:05:52,340
And now I have
lots of slits now.

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00:05:52,340 --> 00:05:55,750
And, again, the
intensity is high,

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00:05:55,750 --> 00:05:57,190
so I can't really
tell how narrow,

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00:05:57,190 --> 00:05:58,840
but I know this
looks very narrow.

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00:05:58,840 --> 00:06:02,882
So maybe we can cut down
the sensitivity a little bit

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00:06:02,882 --> 00:06:03,465
on the camera.

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00:06:08,220 --> 00:06:12,870
And let's see if we get a feel
of how narrow these spots will

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00:06:12,870 --> 00:06:15,030
be.

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00:06:15,030 --> 00:06:20,520
Again, all I can say,
they look pretty narrow.

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00:06:20,520 --> 00:06:24,210
And I'll leave it
to you to calculate

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00:06:24,210 --> 00:06:27,300
how narrow they become.

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00:06:27,300 --> 00:06:28,800
Because I've given
you all the data.

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00:06:28,800 --> 00:06:31,770
I've given you the
spacing between the slits,

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00:06:31,770 --> 00:06:33,720
I've given you the
width of the slits,

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00:06:33,720 --> 00:06:36,240
and the wavelength of the
light, and the distance

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00:06:36,240 --> 00:06:39,810
between the slits
and the screen.

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00:06:39,810 --> 00:06:43,590
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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00:06:47,400 --> 00:06:52,680
contributes to the Fraunhofer
diffraction pattern.

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00:06:52,680 --> 00:06:55,890
Now, we're going to look
at multi-slit diffraction

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00:06:55,890 --> 00:07:00,870
as a function of line spacing.

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00:07:00,870 --> 00:07:05,790
What we have here
are Ronchi rulings,

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00:07:05,790 --> 00:07:12,310
which are pieces of glass with
lots of lines drawn on them.

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00:07:12,310 --> 00:07:17,500
This one here has about
100 lines per inch.

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00:07:17,500 --> 00:07:22,752
So we're going to put this
Ronchi ruling in here,

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00:07:22,752 --> 00:07:25,490
in our setup.

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00:07:25,490 --> 00:07:31,140
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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00:07:37,590 --> 00:07:42,850
of the lines as possible.

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00:07:42,850 --> 00:07:47,220
We've also added this
attenuator here so

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00:07:47,220 --> 00:07:53,510
that we can adjust the intensity
of the light when we need to.

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00:07:53,510 --> 00:07:56,560
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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00:08:05,560 --> 00:08:11,210
As you can see, there are
plenty of very narrow dots.

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00:08:11,210 --> 00:08:13,540
And, in fact, if you want to
get a feel for this scale,

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00:08:13,540 --> 00:08:19,720
the little circles just
below the diffraction pattern

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00:08:19,720 --> 00:08:24,110
are the 5 centimeter markers
that we've had before.

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00:08:24,110 --> 00:08:28,180
Now, if I attenuate the
intensity a little bit,

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00:08:28,180 --> 00:08:31,930
you can see that the
ones in the center

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are the brightest, of course.

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00:08:34,929 --> 00:08:38,390
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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00:08:46,240 --> 00:08:49,300
and the spacing between
the Ronchi ruling

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00:08:49,300 --> 00:08:52,390
and the screen is,
again, 200 centimeters,

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00:08:52,390 --> 00:08:56,680
and the wave length
is 6328 angstroms,

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00:08:56,680 --> 00:09:03,070
you should be able to check on
the spacing between the fringes

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00:09:03,070 --> 00:09:06,430
and also on their widths.

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00:09:06,430 --> 00:09:09,040
So here they are when
I overexpose them.

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00:09:09,040 --> 00:09:13,300
So we can see the ones
way out in the wings.

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00:09:13,300 --> 00:09:15,940
So this is then the
diffraction pattern--

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00:09:15,940 --> 00:09:18,070
the Fraunhofer
diffraction pattern--

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00:09:18,070 --> 00:09:22,160
associated with a Ronchi
ruling of 100 lines per inch.

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00:09:22,160 --> 00:09:26,650
Now, let's look at
200 lines per inch.

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00:09:26,650 --> 00:09:31,920
So here is the 200
lines per inch.

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00:09:31,920 --> 00:09:34,930
And you can see that the
spacing now is different.

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00:09:34,930 --> 00:09:38,310
But I leave it to
you to check on it.

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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

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00:09:47,380 --> 00:09:49,838
these dots are.

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00:09:49,838 --> 00:09:51,630
They're indeed very
bright, because they're

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saturating our camera.

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00:09:54,310 --> 00:09:56,740
So that's for then
200 lines per inch.

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00:09:56,740 --> 00:10:02,470
Now we go to 300 lines per inch.

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Again, the spacing is different.

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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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00:10:22,110 --> 00:10:25,470
So that's then for
300 lines per inch.

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00:10:25,470 --> 00:10:31,110
The next one is
2,000 lines per inch.

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00:10:31,110 --> 00:10:33,780
Now, when I put
it over here, you

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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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00:10:41,123 --> 00:10:42,540
And, again, that
gives you a check

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00:10:42,540 --> 00:10:48,887
on the number of lines per
centimeter or per inch,

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00:10:48,887 --> 00:10:49,470
as we have it.

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00:10:49,470 --> 00:10:50,137
Now, let me see.

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00:10:50,137 --> 00:10:53,310
If we pull back a little bit--

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00:10:53,310 --> 00:10:58,590
pull back on the camera--
to see the other dots.

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00:10:58,590 --> 00:10:59,850
Yes, here they are.

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00:10:59,850 --> 00:11:02,070
But they're so widely
space, that it's

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00:11:02,070 --> 00:11:06,180
difficult to get them all
on the camera at once.

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00:11:06,180 --> 00:11:09,225
So if we go back to
the original position.

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00:11:09,225 --> 00:11:12,130
If we go in again.

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00:11:12,130 --> 00:11:12,930
Here we are.

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00:11:12,930 --> 00:11:16,120
And now I'm going to
again reduce intensity.

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00:11:16,120 --> 00:11:21,060
You can see how
narrow the spots are.

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00:11:21,060 --> 00:11:27,120
So this then sums up
multi-slit diffraction pattern

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00:11:27,120 --> 00:11:31,290
as a function of line spacing.

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00:11:31,290 --> 00:11:34,410
In the next
demonstration, we're going

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00:11:34,410 --> 00:11:37,710
to show the opposite effect.

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00:11:37,710 --> 00:11:43,685
Instead of slits, we're
going to use thin wires.

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00:11:43,685 --> 00:11:45,060
And then when we
come back, we'll

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00:11:45,060 --> 00:11:47,400
show you what the Fraunhofter
diffraction pattern

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00:11:47,400 --> 00:11:51,350
for very thin wires looks like.