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One of the fundamental properties of
waves, light

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waves, or any other kind of wave, is
interference.

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The idea that a wave, since it is an
oscillating positive

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and negative signal, can interfere with
another wave, of the same wavelength.

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So if the two positives of these two waves
superpose

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on top of each other, then the result is
more intensity.

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If the positive of one superposes on the

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negative of the other, that's called
destructive interference.

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And then you get a, a null.

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Or a minimum of the intensity.

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Ordinarily, this is a little bit difficult
to

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observe in your everyday life with light
waves.

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But, we have here, a carefully controlled
situation, where we can demonstrate that.

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What we've built is an interferometer,
which

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is diagrammed on the board, behind me,
here.

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You can see that we're taking our green
laser beam And

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we're splitting it into two approximately
equal pieces, using a beam splitter.

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So half the light transmits through, and
half the light reflects off.

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Then these two beams

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travel to two different mirrors.

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And then return back to the beam splitter,
and recombine at the beam splitter.

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So now, that recombined beam is what you
see.

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That spot on the wall over there.

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So if I block one arm of the

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interferometer, you see the other beam
over there.

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And if I block the other one, you see the
first one.

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Right, and you won't see any interference
if one or the other is blocked.

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But if we superimpose the two of them,
what

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we see is, at certain points within that
bright circle,

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we see a dark stripe through the middle,
and that dark stripe

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is due to the destructive interference of
one beam against the other beam.

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Now, this destructive interference is
extraordinarily sensitive to the path

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length that one beam has traveled compared
to the other one.

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And, what that means is, that if we shift
one beam of

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the interferometer, the length of one arm,
by just half a wave length.

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Then, that goes from constructive
interference to destructive interference.

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So, we'll shift

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the interference pattern.

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So, an interferometer is incredibly
sensitive to very small motions, because,

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remember, the wavelength of light is only
a few hundred nanometers.

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So if I move this mirror by just a 100
nanometers or so, it will lead

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to a shift in the interference pattern,
and I can do that just by blowing on it.

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Interferometers are incredibly

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sensitive devices for measuring very, very
small changes in the positions of objects.