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SHAOUL EZEKIEL:
Now we're all set

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to look at the Fraunhofer
diffraction pattern associated

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with the two-dimensional
multi slits.

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The setup, again just to
remind you, same as before--

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here's the laser.

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Here's the beam from the
laser reflected by this mirror

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and reflected by this
mirror into a lens.

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Now, the lens here is used
to expand the beam so that we

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can illuminate quite a chunk
of a two-dimensional multiple

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slits.

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And then the diffracted light
then goes on onto the screen.

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Now let's look at the
two-dimensional multiple slits.

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What we have for you--

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we have two Ronchi rulings.

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Each one has so
many lines per inch.

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I'm not going to
tell you because I'm

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going to leave that,
again, as an exercise.

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So we have two identical Ronchi
rulings that are crossed.

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The first one is fixed,
which is over here.

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And the second one is
attached to a rotation

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stage behind the first.

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So we can rotate the
second Ronchi ruling,

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and then we can see what it
does to the diffraction pattern.

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So now let's look at the screen.

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And then as we--

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and as you can see
on the screen now,

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you see the two-dimensional
diffraction pattern

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of multiple slits.

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You can see that they look
different than the single slit.

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We have a lot more dots.

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And again, as I say,
I'm going to leave it

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as an exercise for you
to figure them all out.

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Again, I'd like
to draw attention

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to the dots around here,
to all these cross terms

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in this pattern.

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So this pattern, then, is
associated with the rulings

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crossed or orthogonal.

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Now what I'm going
to do, I'm going

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to rotate the Ronchi ruling
or the multiple slits

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in the back of the fixed one.

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See?

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So again you can see
that the pattern rotates.

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Now, if we get rid of
the insert and then look

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at the entire
pattern, now you can

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see what happens as I rotate
the Ronchi ruling in the back.

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Here we are.

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I could rotate the other way.

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And I hope you can see all
the weak spots, which are

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the cross terms in the pattern.

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Now, in order for you to
calculate the line spacings

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in the Ronchi rulings,
I will give you

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the information you need.

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We have a plane wave that
impinges on the Ronchi ruling.

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And the diameter of it is about
a little over a centimeter.

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The screen is about
100 centimeters away

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from the Ronchi rulings.

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And the wavelength of the
light, as before, is 6328.

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And with all this
information, you

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should be able to calculate
the number of lines

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per inch or millimeter
of the Ronchi ruling.

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But you need still
one more information,

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and that is the
scale on the screen.

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And I'm not going
to put a scale on.

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But I will tell you
that the separation

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between these dots here,
these pair of dots,

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is about 6 millimeters.

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So now, you have
all the information

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you need to calculate the
spacing between the lines

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in the Ronchi ruling.

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Now, just before we quit,
if we can pull away--

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the camera pull away-- and show
you the extent of the pattern,

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now you can see the pattern
extends quite a bit.

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And now we can come in again and
back to where we were before.

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This completes
our demonstrations

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of two-dimensional Fraunhofer
diffraction pattern.

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Next, what we have for you
is Fresnel diffraction.

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

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rearranged so we can look
at some Fresnel diffraction

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patterns.