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SHAOUL EZEKIEL: In our
previous demonstrations

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in two-beam interferometry, we
showed some very good contrast

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in the fringes of the
interference path.

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In this demonstration,
we're going

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to show how easy it is to ruin
the contrast in the fringes.

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The set up we'll use is, again,
a Michaelson interferometer.

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And here it is.

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There's the laser.

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Here's the beam from the laser.

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Now reflect off
this mirror here.

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And then reflect it,
again, by this mirror here,

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into the Michaelson
interferometer.

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Here is one arm.

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Here's the other arm.

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The output of the
interferometer gets

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reflected off this mirror, into
this lens, onto the screen.

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So now let's take a
close look at the screen,

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and look at the fringes.

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Now as you can see, the
contrast between bright and dark

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is pretty good which, means
the dark is pretty dark,

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and the bright is as
bright as we can--

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as we can get it.

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Now I'm going to ruin the
contrast between the fringes

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by simply shaking
this mirror here.

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So you can see that the
contrast is washed out,

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due to the fact that
the fringes are moving.

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If I take my hand away,
the fringes come back.

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I can also tap the
beam splitter here.

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And, again, looks as if
there's no interference at all,

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because I'm shaking
one of the mounts.

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I could also tap the table.

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And, again, I can
wash the fringes out.

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So the conclusion
here is that, if you

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want to get good contrast
between the fringes,

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make sure that
the optical mounts

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within the interferometer
are stable.

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Because if they shake, then
the fringes would shake,

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and you get very
poor contrast or--

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you don't even see
any fringes at all.

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So, then, be careful
how you set up

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an interferometer
so that you can

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get the best contrast possible.